Biguanide derivatives, useful as chemotherapeutic agents or as intermediates therefor, and of the general formula <FORM:0603070/IV(b)/1> wherein R and R\sv each represent a hydrocarbon radical, at least one being aryl, R" and R\sv" each represent hydrogen or an alkyl radical, and of X and Y one represents hydrogen and the other hydrogen or an alkyl radical, provided that at least two of X, Y, R" and R\sv" represent hydrogen and that any or all of the radicals may bear non-acidic substituents, are manufactured by the interaction of a guanidine derivative RR"N-C(=NX)-NH2 with an S-substituted isothiourea Alk S-C(=NY)-NR\svR\sv" (where Alk represents an alkyl or substituted alkyl group, e.g. methyl, ethyl or benzyl), or, in the presence of a desulphurizing agent, with a thiourea YNH-CS-NR\svR\sv". The reaction may be effected by heating the reagents together, advantageously in the presence of a solvent or diluent, e.g. methanol, ethanol or b -ethoxyethanol. The thiourea or S-alkylisothiourea and the guanidine may be used as such, or in the form of salts, in which case the base may be liberated in situ by the addition of a basifying agent, e.g. sodium ethoxide or methoxide. Suitable desulphurizing agents (the use of which is essential with the thioureas and optional but sometimes advantageous with the S-alkylisothioureas) are the oxides and salts of the heavy metals, particularly of lead, copper, silver and mercury. The products may be converted into salts with organic or inorganic acids, e.g. acetic, lactic, methanesulphonic, methylenedisalicylic, methylene-bis-2 : 3-hydroxynaphthoic or hydrochloric acids, by dissolving the biguanides in aqueous solutions of the acids and evaporating off the water, or more conveniently by mixing the components in an organic solvent such as acetone, or an alcohol in which the salts are sparingly soluble. In examples: (1) isopropylthiourea is refluxed with ethyl iodide and ethanol, and the resulting S-ethyl-N-isopropyl-isothiourea hydriodide is refluxed with p-chlorophenylguanidine in ethanol to produce N1-p-chlorophenyl-N5-isopropylbiguanide, which is isolated in the form of its acetate; (2) S-benzyl-N-isopropylisothiourea hydrochloride similarly yields the same product (isolated as the hydrochloride); (3) S-isopropyl-N-isopropylisothiourea hydrobromide similarly gives the same product (as acetate); (4) the p-chlorophenylguanidine in (3) is replaced by phenylguanidine carbonate, producing N1-phenyl-N5-isopropylbiguanide (as hydrochloride); (5) S-ethyl-N-ethylisothiourea hydrobromide is treated as in (3) to form N1-p-chlorophenyl-N5-ethylbiguanide acetate; (6) S-ethyl-N-isopropylisothiourea hydrobromide similarly yields the same product as in (3); there may similarly be obtained from the appropriate intermediates: N1-p-chlorophenyl-N5-n-propyl- and -N5-ethyl-biguanide, and (as hydrochlorides) N1-p-bromo- and -iodo-, -m-chloro-, bromo- and -iodo-, -3 : 4- and -3 : 5-dichloro-, -3 : 4-dibromo- and -di-iodo-, -3 : 4 : 5-trichloro-, - 3 - chloro - bromo-, - iodo- and -methyl-, -4-chloro-3-methyl-, -bromo- and -iodo-, -4-iodo-3-bromo-, -3:5-dichloro-4-bromo-and - 4 - bromo - 3 - iodo - phenyl - N5 - isopropylbiguanide, N1-p-bromo- and -iodo-, -3-chloro-4-bromo- and -iodo- and -4-chloro-3-iodo-phenyl-N5-ethylbiguanide, N1-p-bromophenyl-N5-n-propyl- and -N5-sec.-butyl-biguanide, N1-p-chloro-, -bromo- and -iodo- and -3-chloro-4-bromo- and -iodo-phenyl-N5-methyl-N5-isopropylbiguanide, N1-p-chloro- and -3-chloro-4-bromo-phenyl-N5-methyl-N5-n -propylbiguanide, N1-m-chloro- and -bromo-, -p-iodo- and -3-chloro-4-bromo- and -iodo-phenyl-N5-n-propylbiguanide, and N1-p-iodophenyl-N5-n-butylbiguanide; (7) p-ethoxyphenylguanidine and S - ethyl - N - isopropylisothiourea hydrobromide similarly yield N1-p-ethoxyphenyl-N5-isopropylbiguanide (as hydrochloride); (8) p-chlorophenylguanidine and isopropylthiourea are refluxed in ethanol in the presence of litharge to form the product of (1); there may similarly be made the other products of (6); (9) p-methoxyphenylguanidine is similarly treated, but using mercuric oxide instead of litharge, producing N1 - p - methoxyphenyl - N5 - isopropylbiguanide (as hydrochloride); (10) isopropylguanidine sulphate is stirred with sodium in acetone and the product with N-p-chlorophenylthiourea and mercuric oxide to give the product of (1); from appropriate starting materials there may similarly be obtained a number of the biguanides of (6); (11) the isopropylthiourea in (8) is replaced by p-chlorophenylthiourea, yielding N1 : N5 - di - (p - chlorophenyl) - biguanide (as hydrochloride); (12) isopropyl-guanidine sulphate is stirred with alcoholic caustic soda and the product with N1-p-chlorophenyl-N2-methylthiourea and mercuric oxide to give N1-p-chlorophenyl-N2-methyl-N5-iso propylbiguanide (as dihydrochloride monohydrate); from the appropriate intermediates there may similarly be prepared the hydrochlorides of: N1-p-chlorophenyl-N2-ethyl-N5-isopropyl-, -N2-methyl-N5-ethyl-, -N2 : N5 dimethyl- and -diethyl-, -N2-ethyl-N5-methyl- and -n - propyl- and -N2 - methyl - N5 - n - propyl - biguanide, and N1-p-bromo- and -iodo-henpyl-N2-methyl- and -ethyl-N5-isopropylbiguanide; (13) N : N-dimethylguanidine sulphate is stirred with a solution of sodium in ethanol and the product with p-chlorophenylthiourea and mercuric oxide to produce N1-p-chlorophenyl-N5 : N5-dimethylbiguanide; from the appropriate intermediates there may similarly be obtained : N1-p-chloro-, -bromo- and -iodo- and -3-chloro-4-bromo- and -iodo-phenyl-N5-methyl-N5-isopropylbiguanide, and N1-p-chloro- and - 3 - chloro - 4 - bromo - phenyl - N5 - methyl - N5 - n - propylbiguanide; (14) isopropylguanidine sulphate is stirred with alcoholic caustic soda and the product refluxed with N-p-chlorophenyl-S-methylisothiourea to give N1-p-chlorophenyl-N5-isopropylbiguanide (as acetate); there may similarly be made the other products of (6). Additional starting materials specified are, on the one hand, p-tolyl-, p-nitrophenyl-, N : N-phenylmethyl-, 3 : 5 - dichloro - 4 - iodophenyl-, methyl-, cyclohexyl-, N : N-diethyl- and N : N-methyl-n-propyl-guanidine, and, on the other hand, 3 : 5-dichloro - 4 - iodophenyl-, N-methyl-and N : N-diethyl-thiourea and the corresponds ing S-alkylisothioureas. Guanidine derivatives of the general formula given above are obtainable by interaction of the appropriate amine hydrochloride with cyanamide, or, when not containing aryl radical,-by interaction of an alkylamine with S-methylisothiourea. N-Arylthioureas are obtainable by interaction of an arylamine hydrochloride with ammonium thiocyanate. N-Alkylthioureas are obtainable by interaction of an alkyl isothiocyanate with ammonia. N : N-Dialkylthioureas are obtainable by the action of hydrogen sulphide on a dialkylcyanamide. S-Substituted isothioureas are obtainable by alkylation of the corresponding thioureas, e.g. by the action of benzyl chloride, isopropyl bromide or ethyl bromide on N-isopropylthiourea or of ethyl bromide on N-ethylthiourea to give the starting materials of examples (2), (3), (6) and (5) respectively.