The wool fibre contains a fatty acid component which can only be liberated from the fibre by treatment with alcoholic alkali solutions. The major fatty acid from this component has been isolated in quantity and purified. Using GC/MS and NMR, the fatty acid was identified as 18-methyleicosanoic acid. The results obtained from transesterification experiments suggest that the fatty acid is covalently bound to the fibre protein by an ester linkage.