Two conjugates (HIM6-PAD-ara C and HIM6-PLGA-ara C) of the anticancer agent cytosine arabinoside (ara C) and monoclonal antibody (HIM6) against human leukocytes were prepared with dextran T-40 and poly-L-glutamic acid as intermediate carriers, respectively. The drug-antibody conjugates maintained most of the original antigen-binding activity of the free antibody. The ratio of positive bound cells was found to be > 90% by an indirect immunofluorescence assay. The cytotoxicities of HIM6-PAD-ara C and HIM6-PLGA-ara C against antibody-reactive human leukemia HL60 cells were lower than those of free ara C and a mixture of ara C and HIM6 (IC50s of HIM6-PAD-ara C, HIM6-PLGA-ara C, ara C and the mixture of ara C and HIM6 were 0.212, 0.102, 0.028 and 0.024 microgram/ml, respectively), but were similar to those of the intermediates PAD-ara C and PLGA-ara C. On the other hand, these two conjugates showed no cytotoxic its against non-target hepatoma cells. These results indicate that the specific cytotoxicity of the conjugate depends on specific binding to the target surface antigen by the monoclonal antibody in the conjugate molecule.