Welwitindolinones are a family of novel alkaloids recently isolated from the blue-green alga Hapalosiphon welwitschii. We demonstrate that incubation of SK-OV-3 human ovarian carcinoma cells and A-10 vascular smooth muscle cells with welwitindolinone C isothiocyanate, now termed welwistatin, results in dose-dependent inhibition of cell proliferation, which is correlated with increases in the percentage of cells in mitosis. Treatment of A-10 cells with welwistatin resulted in reversible depletion of cellular microtubules but did not affect microfilaments. Pretreatment of A-10 cells with paclitaxel prevented microtubule depolymerization in response to welwistatin. Welwistatin inhibited the polymerization of purified tubulin in vitro but did not alter the ability of tubulin to bind [3H]colchicine or to hydrolyze GTP. Also, welwistatin did not induce the formation of topoisomerase/DNA complexes. Results of the present study indicate that welwistatin is a new antimicrotubule compound that circumvents multiple drug resistance and so may be useful in the treatment of drug-resistant tumors.