Wool fatty alcohols (WF-Alc) separated from esterolysates of sheep cutaneous wax were fractionated by molecular distillation and HPLC, resulting in two homogeneous antitumor compounds identified by GC-MS and 13C-/1H-NMR as a-monohydric branched saturated fatty alcohols, 14-methyl-1-pentadecanol (iso-C16OH) and 14-methyl-1-hexadecanol (anteiso-C17OH). Both the alcohols markedly prolonged the life-span of Ehrlich ascites carcinoma- or S180 fibrosarcoma-implanted mice without acute toxic side-effects, whereas other iso- or n-fatty alcohols (C16-C30) contained in WF-Alc were slightly effective. Thus antitumor activity of WF-Alc depended on the molecular balanced hydrophobicity attributed to both the branched alkyl moiety bulkiness derived from the diverse fatty alcohols contained, and a hydroxyl group.