The kinetics of colchicine binding to bovine brain tubulin have been reported to be biphasic under pseudo first order conditions [(1978) Biochemistry 17, 4466-4472]. Unlike brain tubulin, the kinetics of colchicine binding to bovine renal tubulin are monophasic. The apparent on-rate constant for the binding of colchicine to renal tubulin is found to be very close to that of the faster binding component in brain tubulin. Similarly, the dissociation of colchicine-tubulin complex in the presence of iodide is biphasic for brain tubulin but monophasic for renal tubulin. Since brain and renal tubulin apparently differ in beta-tubulin, our results suggest that the biphasic nature of the kinetics for bovine brain tubulin could possibly originate from the existence of multiple isotypes of tubulin differing in drug binding affinity.