The ability of mebendazole and fenbendazole to bind to tubulin in cytosolic fractions from 8-day Ascaris suum embryos was determined by inhibition studies with [3H]colchicine. Colchicine binding in the presence of 1 . 10(-6) M mebendazole was completely inhibited during a 6 h incubation period at 37 degrees C. Inhibition of colchicine binding to A. suum embryonic tubulin by mebendazole and fenbendazole appeared to be noncompetitive. The inhibition constants of mebendazole and fenbendazole for A. suum embryonic tubulin were 1.9 . 10(-8) M and 6.5 . 10(-8) M, respectively. Mebendazole and fenbendazole appeared to be competitive inhibitors of colchicine binding to bovine brain tubulin. The inhibition constants of mebendazole and fenbendazole for bovine brain tubulin were 7.3 . 10(-6) M and 1.7 . 10(-5) M, respectively. These values are 250-400 times greater than the inhibition constants of fenbendazole and mebendazole for A. suum embryonic tubulin. Differential binding affinities between nematode tubulin and mammalian tubulin for benzimidazoles may explain the selective toxicity. The importance of tubulin as a receptor for anthelmintic benzimidazoles in animal parasitic nematodes is discussed.