Abstract In transplantable tumor cell lines, a common mode of resistance to several natural products, including the anthracyclines, the Vinca alkaloids and actinomycin D, involves an outward transport process which limits drug accumulation to sub-lethal levels (1–5). This transport system can be inhibited, and drug responsiveness promoted, by calcium antagonists of two structural classes: papaverine analogs, e.g. verapamil, and dihydropyrimidines, e.g. nitrendipine (6–9). This report describes studies on the two classes of calcium antagonists named above. The data indicate that verapamil is a substrate for the outward transport system and promotes anthracycline accumulation via competition for exodus. Nitrendipine is not a substrate for this outward transport system and may act via a ‘chaotropic’ mechanism.