We have developed an in vitro immunization system for the production of B-cell lines that secrete HLA-specific human mAbs. For this purpose, peripheral blood lymphocytes of parous women were stimulated with pools of allogeneic lymphocytes. Preferential outgrowth of B-lymphocytes was effected by inclusion of rIL-2 and a B-cell specific nucleoside analogue. Stimulated B cells were immortalized by EBV transformation, and specific antibody-producing transformants were fused to heteromyeloma or mouse myeloma cell lines, yielding stable hybridomas. This approach has led to the successful development of two human heterohybridomas producing HLA-specific mAbs reactive by complement-mediated cytotoxicity. The specificities of these human mAbs, reactive with HLA-B12(44 + 45) and HLA-B60, respectively, are fully concordant with those of HLA-typing sera.