The antimitotic depsipeptide dolastatin 15 was radiolabeled with tritium in its amino-terminal dolavaline residue. Dolastatin 15, although potently cytotoxic, is a relatively weak inhibitor oftubulin assembly and does not inhibit the binding ofany other ligand to tubulin. The only methodo-logy found to demonstrate an interaction between the dep-sipeptide and tubulin was Hummel–Dreyer equilibrium chromatography on Sephadex G-50 superfine. The average apparent Kd value obtained in these studies was about 30lM, with no difference observed when column size or tubulin concentration was varied
