B-cell stimulatory factor-1 (BSF-1)¹, formerly designated B-cell growth factor, is a T-cell-derived factor required for entry into the S phase of the cell cycle by B cells stimulated with low concentrations of anti-IgM antibodies^2–5. BSF-1 acts directly on resting B cells to prepare them to synthesize DNA more promptly on subsequent exposure to competent stimuli^6,7 and to strikingly enhance their expression of class II molecules of the major his-tocompatibility complex^8,9. Previous studies have shown that murine BSF-1 can be separated physically from interleukin-2 (IL-2) and that the molecule has an apparent relative molecular mass (M_r) of ∼ 15,000 and pI values of 6.4–6.7 and 7.4 (ref. 10). Here, we report the production of a monoclonal antibody to BSF-1, its use in characterizing BSF-1, and functional studies demonstrating that this molecule is distinct from IL-1, IL-2 and IL-3.