An elastic thread is made from an anhydrous liquid polyurethane prepolymer which is a reaction product of a polymer having terminal alcoholic hydroxyl groups selected from polyesters and polyethers with an excess of an aromatic, aliphatic or cycloaliphatic diisocyanate, by extruding the prepolymer in the form of a filament, passing the filament into a setting bath of 50-97% by weight of an aliphatic amine having two primary amine groups, and 50%-3% by weight of a hydroxyl-containing solvent to effect a setting of the external surface only of the extruded material to a solid state while the interior of the filament remains fluid, and thereafter setting the interior of the filament to a solid, non-porous state under anhydrous conditions at a temperature between 37 DEG C. and the melting point of the polyurethane. In a preferred method, the polymer is an ethylene-propylene adipate, the diisocyanate is tolylene diisocyanate, the aliphatic amine is ethylene diamine, the hydroxyl-containing solvent is water and the interior of the filament is set at 65-205 DEG C., desirably 80-150 DEG C. In general, the curing agent may consist solely of the diisocyanate present in the liquid prepolymer or may consist of the diisocyanate and an organic chain extender having at least two groups reactive with isocyanate groups. Suitable chain extenders, which may be present in a proportion of 0.25%-2% of the prepolymer, include triisopropanolamine, 1,4-butanediol, triethanolamine, Quadrol(tetrakis N,N1-b -hydroxyethyl ethylene diamine) and diethanolamine. The diisocyanate may be present in a molar excess of 20-250% over the polymer, which may be a polyester of a dicarboxylic acid and a glycol or a polyether containing two hydroxyl groups, made by a process in which the polymer-forming reactants include a small amount of a trihydric alcohol, e.g. a polyester may be made from a mixture of ethylene glycol, propylene glycol, adipic acid and a small amount of trimethylol propane or trimethylolethane. Other specified glycols include those with 2-20 carbon atoms, e.g. trimethylene glycol, 1,4-butylene glycol, 1,6-hexanediol; 1,4-butenediol, neopentyl glycol, diethylene glycol, thiodiglycol, and 2,21-dimethyl-1, 3-propylene glycol. Suitable acids include those with 4-20 carbon atoms, e.g. succinic, maleic, dihydromuconic, thiodipropionic, adipic, methyl adipic, glutaric, dimerized linoleic, sebacic, suberic, phthalic, and terephthalic acids. Hydroxycarboxylic acids or their lactones may also be used e.g. caprolactone. Less suitably, natural polyesters may be used e.g. castor oil as well as blown drying oils e.g. blown tung, linseed and soya oils. An excess of the glycol over the acid is used in making the polyesters so that they contain terminal hydroxyl groups. Polyethers containing terminal hydroxyl groups which may be used in place of the polyesters include polyethylene glycol, polypropylene glycol, mixed polyethylene-polypropylene glycol and polytetramethylene glycol. Polyethers may be used in conjunction with polyesters as in for example polydiethylene glycol adipate and polytriethylene glycol adipate. The polyester or polyether is "capped" with an excess of the diisocyanate by mixing the reagents at room or elevated temperatures e.g. 70-150 DEG C. and especially 90-100 DEG C. to form an uncured liquid prepolymer soluble in methyl ethyl ketone, the prepolymer being essentially a linear polyurethane having terminal isocyanate groups. In addition to tolylene diisocyanate, others specified include m- and p-phenylene diisocyanate, p-p1-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, p-p1-diphenyl methane diisocyanate, 3,31-bitolyene-4,41-diisocyanate 2,4-6-tolylene diisocyanate dimer, dianisidine diisocyanate, and 4-chloro-1,3-phenylene diisocyanate. Prior to spinning the polyester or polyether and diisocyanate mixture may be diluted with 0-20% of conventional urethane solvents e.g. acetone, methyl ethyl ketone, dimethyl formamide, dimethyl acetamide, ethylene carbonate, propylene carbonate and dimethyl sulphoxide. Pigments, delusterants, heat stabilisers and ultra-violet absorbents may also be added. Suitable amines for the amine-water bath include, in addition to ethylene diamine, hexamethylene diamine, diethylene triamine, tetraethylene pentamine, 3,31-diaminodipropyl ether, diaminodibutyl sulphide and propylene diamine. The preferred hydroxyl compound is water, but other compounds which may be used include ethylene glycol, 1,4-butanediol, 1,3-butylene glycol, propylene glycol, glycerine, hexanetriol and ethyl alcohol. Desirably the bath is at 90-95 DEG C., but room temperature or temperatures of 35-100 DEG C. may in general be used. The surface of the extruded filament quickly becomes solid, the interior remaining liquid, and further diffusion of the diamine into the core is thus prevented. After spinning, and prior to curing the interior, the thread may be passed first through a water bath to remove unreacted diamine, dried over steam heated rolls and then passed through a talc box to coat the surface of the thread and eliminate the possibility of sticking to itself when taken up on a spool. The threads are then cured e.g. at 65-205 DEG C. for 30 minutes to 30 hours, usually 1-18 hours, preferably at 90-120 DEG C. for 1-2 hours. The excess diisocyanate present in the core reacts internally to complete the cure, so that no curing agent need be present. The cured threads can be stretched up to 400% their original length in conventional manner, the stretched threads being then post-cured at least 65 DEG C., 93 DEG C.-150 DEG C. being preferred, for 30 minutes-30 hours, usually for 1-15 hours. The elastic polyurethane threads so made by the partial interfacial and bulk polymerization process are thought to comprise polyester or polyether segments capped with a molecular excess of diisocyanates and reacted with aliphatic primary diamines, biuret linkages obtained from subsequent reaction of the urea hydrogens with the excess diisocyanate, and allophonate linkages obtained from subsequent reaction of the urethane hydrogens with the excess diisocyanate. The threads show no blistering, and do not discolour upon heat ageing, and are resistant to chlorine bleach. U.S.A. Specifications 2,650,212, 2,755,266, 2,953,839, 2,957,852 and 2,962,470 are referred to.