A strategy for cloning human immunoglobulin genes with antigen in vitro immunization and RT-PCR was established. Colorectal carcinoma-associated antigen CA-Hb3 were purified through a monoclonal antibody Hb3-Sepharose 4B affinity column and analyzed by SDS-PAGE and Western blot. Peripheral blood lymphocytes (PBL) from 10 ml blood of a patient with colorectal carcinoma were immunized with CA-Hb3, rhIL-2 and pokeweed mitogen. The lymphoblast-like cells and colonies could been seen after immunization. The amounts of total RNA from the immunized PBL are 2.5 times more than that from the non-immunized. Human immunoglobulin, VH-CH1 (IgG) and VL-CL (κ), genes were amplified with RT-PCR with the above total RNA or mRNA as templates. The products of human immunoglobulin genes from the immunized PBL are 1.3 times more than that from the non-immunized. This strategy would be used for humanizing mouse-original monoclonal antibodies.