B cell growth and differentiation into immunoglobulin secreting cells is controlled by various cytokines and cell to cell contact with T cells. Fusion partner for human hybridoma therefore should accommodate all or some of these signaling systems to overcome the unique situation of MHC incompatibility, need for specific growth factors simultaneously taking into consideration the downstream processing of the product for the clinical use. We have thus directed our efforts towards the development of a fusion partner which would not need Epstein-Barr virus transformation of B cells prior to fusion. A nontransforming mitogen, formalinized Staphylococcus aureus (FSTA) was used for stimulating human B cells. Successful production of human IgM monoclonal antibody was achieved by incorporating Jurkat-4 cells in existing mouse human heterohybrid through fusion of these cells followed by fusion with human B cells. To accommodate chromosomes of both T and B cells after fusion, human myeloid precursor cells KG1a, and to incorporate T cell, HuT78 cells were fused. CD34+ and CD4+ hybrid of KG1a and HuT 78 cells-434 AM-when used as fusion partner could allow secretion of MAbs, however growth potential was low. SP2/0 cells were then incorporated in 434 AM cells to give myeloma environment to fused human B cells. Rabies virus neutralizing human IgG MAb secreting clone was generated by fusing FSTA stimulated human B cells with this fusion partner.