The critical surface tension of wool fibers was measured using the sink-float technique with various classes of tests liquids. Nonpolar organic liquids and aqueous surfactants gave similar values of ?C, but much lower values were obtained with butanol/water mixtures. Adsorption of butanol on the surface of the fibers is thought to be responsible for the low values of ?C in the latter case. Removal of lipid material from the surface of the fibers by extraction increases ?C from 30 mN/m to 37 mN/m, similar to that obtained for horn keratin or polypeptide material. Exposure of fibers to a corona discharge in air at atmospheric pressure also increases ?C.