Coronaridine [1], conoduramine [2], and voacamine [3], three indole alkaloids isolated from Peschiera laeta, have been found to enhance the cytotoxic response mediated by vinblastine [4] with multidrug-resistant KB cells. Inhibition of vinblastine binding with membrane vesicles isolated from this cell line was also assessed, and the bisindole alkaloids conoduramine [2] and voacamine [3] were found to be more potent inhibitory agents than the monomeric alkaloid, coronaridine [1]. Thus, these compounds appear to function by binding with P-glycoprotein.