A noncytotoxic dose of verapamil (Ver) 3 micrograms/ml was found to potentiate 10-fold the growth-inhibitory effects of adriamycin (ADM) in ADM-resistant Chinese hamster ovary cell line (RC1). Ver 3 micrograms/ml also reduced the IC50 value of ADM from 1.2 micrograms/ml to 0.08 microgram/ml in RC1 in clonogenic assay. The index of reversing resistance was 15-fold. In an attempt to elucidate the mechanism of the reversion of the multidrug resistance by Ver, Ver in combination with ADM was found to enhance intracellular ADM accumulation in RC1, and brought about more toxicity in RC1 than ADM alone; and Ver in combination with ADM was shown to retard RC1 in G2 + M phase by flow cytometry analysis.