Long-circulating liposomes loaded with doxorubicin (Dox) were additionally modified with the prostate cell-specific monoclonal antibody 5D4 (mAb 5D4). The resultant Dox-loaded 5D4-immunoliposomes specifically recognized prostate cancer cell lines of several different types expressing the mAb 5D4 antigen, PSMA, and significantly enhanced cytotoxicity toward these cells compared with the non-targeted Dox-liposomes in vitro while no increased toxicity was observed toward non-prostate (lung) cancer cell line.