An intracellular adriamycin (ADM)-binding protein purified from the cytosol of L1210 mouse lymphocytic leukemia cells had a molecular weight of 700-1500 kDa and hydrolyzed Suc-LLVY-MCA. When L1210 cells were incubated with a photoactive A]DM analogue, N-(p-azidobenzoyl)-adriamycin (NAB-ADM), most of the NAB-ADM was found to localize in the nuclei. In situ photoaffinity labeling of L1210 cells with NAB-ADM resulted in low protease activity in the cytosol and nuclear extracts and the cells showed selective photoincorporation of NAB-ADM into the proteasome, These results suggest that the proteasome is a translocator of ADM from the cytoplasm to the nucleus and might therefore become a new candidate for cancer chemotherapy.