Résumé : We investigated the cytotoxicity of adriamycin in two human ovarian tumor cell lines, AZ224 and AZ364, by the MTT-test and we analysed the formation of DNA single-strand (SSB) and double-strand breaks (DSB) by means of the alkaline and neutral elution technique. The AZ364 cell line was 15 times more resistant to ADR (ID 50 = 10.0 μg/ml) than the AZ224 cell line (ID 50 = 0.66 μg/ml) after 1 hr of drug exposure. Immediately after exposure, we observed a biphasic dose response for SSB in the AZ224 cells over a concentration range of 0.1 to 32.0 μg/ml, while practically no DSB were found. Upon drug removal and incubation in drug-free medium, full repair of SSB was observed for an ADR concentration of 1 μg/ml. On the contrary, the DSB became significantly increased for all tested concentrations and persisted as observed after 3 hr and 7 hr in drug-free medium. The resistant cell line AZ 364 showed consistently less DNA breakage than the AZ 224 cell line. This inherent difference in sensitivity to ADR could, however, not be explained on the basis of the cellular pharmacokinetics of the drug. Verapamil induced a 3 to 4 fold potentiation of the ADR cytotoxicity in both cell lines after continuous exposure and was associated with an increase in DNA-breakage. The results of our study confirm that there is a lack of correlation between cytotoxicity of ADR and DNA strand breakage immediately after 1 hr of drug exposure. Instead, we emphasize the importance of the formation, extent and persistence of protein-associated DSB upon drug removal to the cytotoxic action of ADR in vitro.