A subline highly resistant to Adriamycin (SBC-3/ADM100) was isolated in vitro from the human small cell lung cancer cell line, SBC-3, by culturing in progressively higher concentrations of Adriamycin. The SBC-3/ADM100 cells were 106-fold more resistant to the drug than the parent cells in an inhibitory concentration of 50% determined by the MTT assay. The population-doubling time was much longer in SBC-3/ADM100 than in the parent cells. Northern blot hybridization revealed marked overexpression of the MDR1 mRNA in the resistant cells. P-glycoprotein overexpression and a decrease in intracellular accumulation of Adriamycin were demonstrated in SBC-3/ADM100, indicating that outward drug transport was the major mechanism of resistance in this subline. Additionally, a significant elevation of the intracellular glutathione content coupled with the glutathione S-transferase (GST) pi level and a decrease in DNA topoisomerase II (Topo II) activity were noted in this resistant subline. These results indicate that the mechanism of resistance to Adriamycin is multifactorial; involving altered growth characteristics, an enhanced outward transport, enhanced drug detoxification process, and decreased target enzyme activity. The resistant subline will serve as a useful tool in the search for ways to overcome drug resistance.