The binding of tritiated benzimidazole carbamates ([3H]BZCs) to mammalian brain tubulin was examined to investigate the kinetics of the BZC-tubulin interaction and to establish the mechanism of the selective toxicity of the BZC based anthelmintics. [3H]BZC binding to tubulin was markedly greater at 4 degrees than at 37 degrees for all ligands. The association constant (Ka) and maximum amount of [3H]BZC bound (Bmax) were temperature dependent for [3H]mebendazole ([3H]MBZ), [3H]oxibendazole ([3H]-OBZ) and [3H]oxfendazole ([3H]OFZ). The Ka and Bmax values obtained for [3H]MBZ, [3H]OBZ and [3H]OFZ, and the comparatively weak binding of [3H]carbendazim, reflected the known in vitro potency of these compounds as microtubule inhibitors. Dissociation of the [3H]MBZ-tubulin complex was also temperature dependent, the first order dissociation rate constant being reduced by two orders of magnitude at 4 degrees compared with that observed for 37 degrees. These results indicate that the binding of BZCs to mammalian brain tubulin is temperature dependent and suggest that temperature induced conformational changes in the tubulin dimer influence the ability of the BZCs to form a stable BZC-tubulin complex. The temperature dependence of BZC binding and the affinity of the BZCs for mammalian tubulin are therefore unlike the BZC-tubulin interaction observed for parasitic nematodes, where optimum BZC binding occurs at 37 degrees and results in the formation of a pseudo-irreversible complex.