From experiments in vitro it is found that cysteine, in the presence of certain oxidizing agents, can act as a powerful catalyst for polymerizing ethylenic monomers. This aids in explaining the reactions which occur when wool is reduced with sodium bisulphite and then treated with a monomer. Polymerization within the fibre is probably initiated by free radicals produced from reduced cystine by the action of sulphoxides or sulphones present in the wool as a result of atmospheric oxidation. Fraction (A+B) of the cystine appears to be involved as pretreatment with alkali, which converts this fraction to lanthionine, prevents polymerization from taking place. Little or no polymer can be formed in wool from the base of a staple, whereas polymer can be synthesized quite readily in the tip where oxidation products are more likely to be present. If, however, wool from the base is pretreated with a mild oxidizing agent, polymerization will then occur as readily as in the tip. The polymer is apparently chemically bound to the wool through thio-ether linkages.