The mechanism of enhancement of Bleomycin A5 antitumor activity by verapamil was explored by flow cytometry and tracing the radiolabelled bleomycin A5 in vivo. Verapamil was found to increase the G2 blocking effect of bleomycin A5 prominently in mouse S-180 and human HEP-2 cell lines. The distribution of 57Co-bleomycin A5 in mice bearing S-180 sarcoma was changed by verapamil and accumulation of the drug in tumor was increased. In contrast, the labelled drug in the lung was decreased. It seems that the effects of verapamil in enhancing the antitumor activity of bleomycin A5 are to increase the accumulation of the drug in tumor cells and enhance the G2 blocking effect of the drug in cell cycle.