The cytotoxic activity of bendamustine hydrochloride was evaluated against human ovarian and breast carcinoma cell lines including cell lines resistant to cisplatin and doxorubicin in vitro. The relative degree of resistance to bendamustine hydrochloride was lower in all cell lines compared with cyclophosphamide, melphalan and BCNU, suggesting only incomplete cross-resistance. Furthermore lower levels of resistance were also observed in all breast cancer cell lines when bendamustine hydrochloride was compared with cisplatin. Bendamustine hydrochloride also presents good activity in cell line MCF 7 AD, which is approximately 80-fold resistant to doxorubicin compared with MCF 7. Basic glutathione levels and activity of glutathione-S-transferase showed no correlation to the IC50 values for bendamustine hydrochloride in the cell lines. When given at equitoxic concentrations, bendamustine hydrochloride consistently induced more DNA double-strand breaks than melphalan, cyclophosphamide or BCNU. In addition, removal of DNA double-strand breaks induced by bendamustine hydrochloride was relatively slow with the majority of DNA double-strand breaks still being detectable after 24 h. These findings indicate differences in the interaction between bendamustine hydrochloride and DNA, and may explain the lack of complete cross-resistance between bendamustine hydrochloride and the other alkylating agents.