At least two distinct B cell stimulatory factors (BSF) were found to be involved in the differentiation of Staphylococcus aureus Cowan I (SAC)-stimulated human B cells to IgG-producing cells. A factor tentatively called B cell differentiation factor I (BCDF I) was found in one fraction, and a second factor, BCDF II was found in another fraction. The BCDF I fraction alone induces IgG-production in SAC-stimulated B cells, but the BCDF II fraction does not. The BCDF II fraction enhances IgG production in SAC-stimulated B cells in the presence of the BCDF I fraction. Studies concerning the time-course of the action of the BCDF II fraction revealed that it contains a late-acting differentiation factor that acts on B cells most effectively when it is added to the SAC-stimulated B cell culture after the addition of BCDF I fraction; it induces IgG plaque-forming cells within 1 day. The pI value of a late-acting BCDF was in the range of 5 to 6; this pI range is different from that of BCDF I but similar to that of BCDF II, which was shown in our previous studies to be able to induce IgG production in Epstein Barr Virus-transformed B lymphoblastoid cell lines. In addition, the m.w. of a late-acting BCDF were about 35,000 and 20,000, which are the same as those of BCDF II, and thus its identity with BCDF II was suggested.