Abstract: AIM　In order to accumulate into its target specifically, the immunoliposomes must possess two characteristics: specific target efficiency to its target cells and prolonged circulation in blood. A new type of polyethylene glycol (PEG)-immunoliposomes carrying monoclonal antibodies at the distal end of PEG chains should be developed. METHODS　A dipalmitoylphosphatidylethanolamine (DPPE) derivative of PEG with carboxyl group (DPPE-PEG₃₀₀₀-COOH) was newly synthesized. Small unilamellar liposomes were prepared from egg phosphatidyl choline and cholesterol (5∶4, mol/mol) containing 6 mol% DPPE-PEG₃₀₀₀-COOH using reverse-phase evaporation method followed with bath sonication. Monoclonal antibody of human bladder cancer cell (BDI-1), which is highly specific to human bladder cancer cell, was conjugated to PEG-liposomes as well as mouse IgG at the distal end of polyethylene glycol chain. Doxorubicin was entrapped into these immunoliposomes by remote (NH₄)₂SO₄ gradient loading method. The specific targeting efficiency of these immnoliposomes was tested by cytotoxicity test in vitro, enzyme-linked immune sorbent assay (ELISA) and indirect fluorescent immunoassay. Its biodistribution was carried out in mice. RESULTS　The specific targeting efficiency of BDI-1 immunoliposomes (BDI-1-IML)to EJ cells has been demonstrated, in contrast to the non-specific human colon carcinoma cells (LOVO). PEG-liposomes linked with mouse IgG (mouse-IgG-immunoliposomes, IgG-IML) displayed lower reticulo-endothelial systems (RES) uptake and longer circulation time than liposomes without PEG after intravenous injection. CONCLUSION　The long circulation of these PEG-immunoliposomes in vivo, combined with its specific targeting efficiency demonstrated in vitro, guarantees the positive targeting efficiency of these immunoliposomes to its target carcinoma in vivo.