Abstract: The immunoconjugate of anti-CCT2 monoclonal antibody linked to bleomycin A6 was adsorbed on colloidal gold particles (McAb-A6-Au). Binding and internalization of McAb-A6-Au particles in human leukemia CEM cells were examined by electron microscopy. After 60 rain at 4℃, McAb-A6-Au particles were bound to the surface membrane of 78% of CEM cells. Transferring to 37℃ for 15 rain, McAb-A6-Au particles were found to be 56% inside the CEM cells and about one third of the cells contained particles in the nucleus.After 4 h at 37℃ the percentage of CEM cells containing McAb-A6-Au particles increased to 72%. However, only 14% of the antigenically irrelevant U937 cells contained these particles and none of them was found in the nucleus. Preincubation with unconjugated anti-CCT2 monoclonal antibody markedly blocked the McAb-A6-Au particle uptake in CEM cells. The McAb-A6-Au particles were internalized through the formation of endocytotic vesicles. In addition, some McAb-A6-Au particles were able to penetrate the plasma membrance directly into eytoplasma and notably into the nucleus. Results indicate that the immunoconjugate of monoclonal antibody linked to bieomycin A6 showed selective binding to target cells and entered the cells specifically and, rapidly.