In case of lymphoid malignancies of B cell origin and Epstein-Barr Virus (EBV) transformed B cells, it is invariably found that these cells proliferate in the apparent absence of signals from T cells. Many of transformed B cells tested appear to be refractory to the proliferative effects of a number of defined cytokines including IL-1, IL-2, IL-4, IL-5, IL-6, IL-7, TNF alpha, lymphotoxin and IFN gamma. We have recently reported on the molecular cloning of a T cell derived human B cell growth factor, BCGF-12kD. In this study, we show that recombinant BCGF-12kD induces proliferation in EBV negative undifferentiated lymphoma B cell line, MC116 and Burkitt's lymphoma B cell line, P3HR1. A mouse monoclonal antibody produced against a BCGF-12kD synthetic peptide neutralizes the BCGF activity present in the MC116 conditioned media as well as inhibits the in vitro growth of MC116. The PCR analysis of the BCGF-12kD gene transcription provides further evidence that MC116 cells express the BCGF-12kD gene, whereas this gene is silent in resting and anti-IgM activated normal human B cells. These data clearly suggest an immunoregulatory role of BCGF-12kD in transformed human B cells.