Lymphocytes from regional lymph nodes of patients with colon cancer were fused with a human lymphoblastoid cell line with or without in vitro immunization. The efficacy of these two protocols for the generation of human monoclonal antibodies against colon cancer was investigated. The hyperplastic lymph nodes adjacent to the tumor were the best source of B lymphocytes. Fusion frequency and the number of tumor-reactive clones were markedly increased when the in vitro immunization protocol was applied prior to fusion. As a stimulant in in vitro immunization, the supernatant of pokeweed mitogen-stimulated T lymphocytes was superior to the supernatant of mixed lymphocytes culture. Carcinoembryonic antigen at 20 micrograms/L seemed to be the optimal dose for in vitro immunization. The reactivities of human monoclonal antibodies thus generated were measured by enzyme-linked immunosorbent assay and confirmed by immunoperoxidase staining. Combining in vitro immunization with lymphocytes of cancer patients may lead to the successful production of clinically useful human monoclonal antibodies.