Lepidopteran insect cells (TN-368) were found to be extremely resistant to the anthracycline, doxorubicin. They were approximately 400 fold more resistant to the cytotoxic effects of doxorubicin compared to a mammalian counterpart; V79 hamster lung fibroblast cells. Doxorubicin accumulated into TN-368 cells and bound to DNA in a similar fashion as the interaction of doxorubicin in V79 cells. However, no doxorubicin-induced stabilized DNA-topoisomerese II complexes could be demonstrated in TN-368 cells (even at the high concentrations of doxorubicin that resulted in cytotoxicity). Therefore, the lack of formation of this critical lesion may be the cause of the marked doxorubicin resistance noted in the TN-368 cells. © 1994 Academic Press, Inc.