The self-association of calf brain tubulin in PG buffer (10(-2) M NaPi and 10(-4) M GTP, pH 7.0) induced by the antimitotic drug vinblastine has been investigated by velocity sedimentation. Schlieren sedimentation patterns were examined at low vinblastine concentrations where the boundary resolves into a bimodal one and at high vinblastine concentration where a single forward-skewed peak prevails. Weight-average sedimentation coefficients of tubulin were determined as a function of protein concentration, the results fitting well a self-association model of an idefinite isodesmic mechanism. This was confirmed by computer simulation of the sedimentation boundary profiles.