Anthracycline antibiotics are very effective neoplastic agents widely used clinically. However, because of their many adverse effects (e.g. cardiotoxicity, leukopenia and alopecia), their clinical use has been limited. In order to minimize their adverse effects in clinical cancer chemotherapy, anthracyclines must be selectively transported into tumor cells. If there are differences in transport characteristics between tumor and normal cells, it should be possible to establish a strategy for selectively delivering anthracyclines to tumor cells on the basis of the differences. In human cultured leukemia HL60 cells, as tumor cells, and human fresh mononuclear cells, as normal cells, doxorubicin, pirarubicin, daunorubicin and idarubicin were incorporated via a common carrier-mediated system, but the carriers were different in the two cell types. In HL60 cells, it was indicated that a nucleoside transport system contributed, at least in part, to the transport of doxorubicin and pirarubicin, but not daunorubicin and idarubicin, and its contribution to pirarubicin transport was found in other tumor cells, i.e. mouse ovarian sarcoma M5076 and Ehrlich ascites carcinoma cells. On the other hand, in mononuclear cells, there was no involvement of a nucleoside transport system for the four anthracyclines examined. Therefore, we thought that with the modification of an anthracycline molecule as a substrate for the nucleoside transport system, the anthracycline could be delivered selectively to tumor cells.