Colchiceine, a closely related structural analog of colchicine possessing a C-ring tropolone, has been shown to be a potent inhibitor of microtubule assembly in vitro (I50 = 20 microM). The mechanism of inhibition is mediated through binding to tubulin (KA = 1.2 +/- 0.7 x 10(4) M-1), although potentially not through the colchicine receptor site. Supporting the hypothesis of an alternate receptor are the observation of colchiceine binding to the isolated colchicine-tubulin complex (KA = 2.2 +/- 1.0 x 10(4) M-1), the poor correlation between the competitive inhibition of colchicine binding (KI = 125 microM) and the inhibition of microtubule assembly, and different structure-activity relationships for colchiceine analogs as compared to the colchicine series.