In vitro incubation of the parasitic nematode Ascaris suum in the presence of 10 microM mebendazole (MBZ) resulted in a complete loss of colchicine binding ability of extracts obtained from the parasite's intestine. Biochemical evidence supported the identification of the colchicine binding receptor in A. suum intestinal extracts as tubulin. This protein was partially purified and found to comprise approximately 0.8% of the soluble intestinal protein. MBZ inhibited colchicine binding to the partially purified tubulin in a competitive manner, the inhibition constant being 4.22 X 10(-6) M. Colchicine binding to porcine brain tubulin was also competitively inhibited by MBZ, exhibiting an inhibition constant of 8.0 X 10(-6) M. [3H]Colchicine binding studies revealed an apparent association constant of A. suum tubulin of 5.88 X 10(4) M(-1). Similar experiments employing [3H]MBZ showed that the extent of MBZ binding to the tubulin up to 10(-5) M was linearly dependent on MBZ concentration. Due to solubility problems the precise association constant for MBZ could not be determined but is apparently less than 10(5) M(-1). In view of the small difference in drug binding abilities between nematode intestinal and mammalian brain tubulin it still remains unclear whether the selective toxicity of MBZ can be solely explained by its interference with the parasite's microtubular system. Further studies reported in this paper suggest that a differential pharmacokinetic behaviour of MBZ between parasite and host may be the essential basis for the difference in drug susceptibility between both biological systems.