Neoplastic cells frequently have an increased number of transferrin receptors. Coupling transferrin to an anti-neoplastic drug has the potential to overcome multidrug resistance (MDR). The purpose of this study was to examine the distribution and action of doxorubicin-transferrin conjugate (DOXTRF) in a leukaemia cell line (HL60), a multidrug-resistant leukaemia cell line (HL60ADR) and a normal tissue cell line (human fibroblasts). The intracellular accumulation of DOX and DOX-TRF was monitored by direct fluorescence. More DOX-TRF than free DOX was delivered to the tumour cells, and consecutively the levels of DNA double-strand breaks and apoptosis increased even in the multidrug-resistant cell line. In the normal tissue cell line, DOX-TRF did not accumulate, and therefore, the levels of DNA double-strand breaks and apoptosis did not increase. Cell viability was determined using the MTT assay. The IC(50) for DOX-TRF was lower than the IC(50) value for the free drug in both leukaemia cell lines. The IC(50) values for the HL60 cells were 0.08 microM for DOX and 0.02 microM for DOX-TRF. The IC(50) values for HL60ADR cells were 7 microM for DOX and 0.035 microM for DOX-TRF. In conclusion, DOX-TRF was able to overcome MDR in the leukaemia cell lines while having only a very limited effect on normal tissue cells.