An immunoglobulin production stimulating factor (IPSF) in human lymphoblastoid Namalwa cells was purified by the serial use of ammonium sulfate fractionation, hydrophobic interaction chromatography and gel filtration, and named IPSF-II alpha. IPSF-II alpha was estimated as a 112 KD protein composed of a 40 KD polypeptide and two 36 KD polypeptides. The 36 KD protein extracted from SDS-polyacrylamide gel showed IPSF activity, but not the 40 KD protein. The IPSF activity was reasonably stable in alkaline but unstable in acidic solution and heat-unstable. In a serum-free medium, IPSF-II alpha stimulated IgM production of human-human and mouse-mouse hybridomas 4-15 and 2-fold, respectively, although its growth stimulatory effect on hybridomas was negligible. The factor did not stimulate IgG production in either human or mouse hybridomas in the same serum-free medium. These results suggested that IPSF-II alpha was a new cellular factor for stimulating IgM productivity of hybridomas.