Binding of two colchicine analogues, desacetamidocolchicine and 2-methoxy-5-(2',3',4'-trimethoxyphenyl) tropone to purified tubulin have been studied. Both analogues bind to tubulin with a significant increase in fluorescence polarization of the drugs in solutions containing tubulin. The Kd for tubulin-drug complexes were found to be 1.25 X 10(-6) M and 1.08 X 10(-6) M for desacetamidocolchicine and 2-methoxy-5-(2',3',4'-trimethoxyphenyl) tropone, respectively. Scatchard analysis of the fluorescence titration curve of drug tubulin interaction also gives the values of stoichiometry and affinity constant. These were 0.8 and 1.6 X 10(6) M-1 for desacetamidocolchicine, and 0.9 and 0.58 X 10(6) M-1 for 2-methoxy-5-(2',3',4'-trimethoxyphenyl) tropone.