Two immunoglobulin production stimulating factors (IPSF) have been found in human Burkitt's lymphoma Namalwa cells. One IPSF named IPSF-II alpha was purified and identified as glyceraldehyde-3-phosphate dehydrogenase as previously reported. We report here purification, identification and characterization of IPSF-II beta. IPSF-II beta was purified by the serial use of ammonium sulfate fractionation, hydrophobic interaction column chromatography, anion-exchange column chromatography and gel filtration. The IPSF-II beta was estimated as a 46 KD monomeric polypeptide by gel filtration and SDS-PAGE. Partial amino acid sequence of the 46 KD protein was analyzed for 26 amino acid residues. The sequence very closely coincided with enolase (EC 4.2.1.11) derived from various origins and, it was completely homologous with that of human enolase alpha-chain. Rabbit muscle enolase stimulated IgM production of hybridoma lines, and IPSF-II beta had the enzymic activity. These results suggested that IPSF-II beta was alpha-enolase or its isozyme. IPSF activities of IPSF-II beta was stable in alkaline conditions whereas the enzymic activity was rapidly lost in alkaline conditions. Though IPSF-II beta stimulated IgM production of both human-human and mouse-mouse hybridoma lines in serum-free condition, it partially suppressed IgE production of mouse-mouse hybridoma lines.