Reversal of acquired resistance to vinca alkaloids and anthracycline antibiotics has been attained in vitro and in vivo by some calcium influx blockers and calmodulin inhibitors. These calcium influx blockers and calmodulin inhibitors enhance the intracellular level of vincristine and doxorubicin in tumor cells, especially in drug-resistant mouse and human tumor cells, by inhibiting their outward transport. Among tumor cell lines established from mice and humans, less sensitive tumor cells became more susceptible to vincristine by use of calcium influx blocker, and inherent resistance of tumor cell lines to vincristine has also been circumvented.