The effect of IL 2 on Staphylococcus aureus Cowan I (SAC)-driven IgG production of human B cells was examined by utilizing chromatographically purified IL 2 (R-IL 2) and the transcription product of the cloned cDNA for human IL 2 purified from recombinant E. coli (G-IL 2). Both preparations of IL 2 by themselves were not enough to induce optimal IgG-production in the SAC-stimulated tonsillar B cell fraction, which was highly enriched for B cells, but effectively induced IgG production in the presence of a subeffective number of T cells or a late-acting B cell differentiation factor (BCDF). In addition, the activity that induced IgG production in the presence of a subeffective number of T cells was absorbed with an IL 2-dependent mouse T cell line. These results clearly indicate that IL 2 has a definite effect on B cell differentiation in this system. Although the mechanisms of this effect remain to be elucidated, a direct effect of IL 2 on B cells may be involved, because the addition of IL 2 along with SAC induced a limited but significant increase of 3H-TdR incorporation in the highly enriched B cell population, which showed very little response to PHA and Con A even in the presence of IL 2, and, as mentioned above, IL 2 induced IgG production in the B cell preparation without any supplement of T cells provided the late-acting BCDF fraction was present in the culture.