Cloned lines of Adriamycin (ADR)-sensitive and -resistant P388 leukemia have been established from single cell cultures. A marker chromosome M1 was found in all cells in the heterogeneous resistant P388/ADR parental line as well as in the cloned resistant lines P388/ADR/3 and P388/ADR/7; a different marker chromosome M2 was present in the heterogeneous sensitive P388 parental line as well as the cloned sensitive line P388/4. Dose-survival studies showed that D0, the dose of Adriamycin reducing survival to 1/e (i.e., 37% of the initial population), was 33 +/- 5 (SE) nM for sensitive P388/4 cells, 169 +/- 17 nM for resistant P388/ADR/3 cells, and 336 +/- 28 nM for the more resistant P388/ADR/7 cells. Drug uptake in sensitive P388/4 cells was 1.6-fold greater than in resistant P388/ADR/3 cells and 2.1-fold greater than in resistant P388/ADR/7 cells. The number of DNA single-strand breaks produced per microM Adriamycin was 131 +/- 9 rad equivalents in sensitive clone 4 cells, 41 +/- 8 rad equivalents in resistant clone 3 cells, and 33 +/- 11 rad equivalents in resistant clone 7 cells. The number of DNA double-strand breaks per microM Adriamycin was 1721 +/- 126 rad equivalents in sensitive cells, 117 +/- 36 rad equivalents in resistant P388/ADR/3 cells, and 194 +/- 16 rad equivalents in resistant P388/ADR/7 cells. Differences in drug uptake were insufficient to explain the higher incidence of DNA single- and double-strand breaks in sensitive cells. These findings strongly support the concept that resistance to Adriamycin in P388 leukemia cells is multifactorial; however, this study did not resolve whether these changes arise from a single pleiotropic mutation or from multiple mutations. In sensitive P388/4 cells the number of DNA single-strand breaks formed could all be attributed to double-strand breaks. However, in both resistant cell lines the level of induction of single-strand breaks was in excess of that due to double-strand breaks, and this excess of single-strand breaks appeared to vary directly with the degree of resistance, being greater in the more resistant clone 7 cells than in the less resistant clone 3 cells. In both sensitive and resistant cell lines the ratio of true single- to double-strand breaks varied inversely with the concentration of Adriamycin. Finally, the cytotoxic activity of Adriamycin appeared to correlate more closely with formation of DNA double-strand breaks than with single-strand lesions.