We established a method of in vitro immunization of human lymphocytes against human cancer cells and efficiently obtained human-human hybridomas producing cancer-specific antibodies using the in vitro immunized lymphocytes. Human lymphocytes were cocultured with human lung cancer cell line A549 for four days in medium containing various immunoactive reagents. In vitro immunization was effectively caused by the addition of OK-432 or muramyl peptides, IL-2, and IL-6, to the coculture of A549 cells and lymphocytes. Although specific antibodies of both IgM and IgG classes were produced by this method, the ratio of IgG to IgM was greater than or equal to 2. The efficiencies of in vitro immunization fluctuated about threefold in lymphocytes derived from several donors. The in vitro immunized lymphocytes were fused with human fusion partner A4H12 cells. Hybridomas producing specific antibodies reactive to A549 cells were efficiently obtained by sequential cell fusion. The efficiency of acquisition of hybridomas producing specific antibodies with the in vitro immunized lymphocytes was about 10-fold higher than that with lymphocytes spontaneously immunized in bodies of lung cancer patients.