To investigate the capacity of human IL-4 to function as a B cell growth factor (BCGF), we studied its ability to promote proliferation of a selected B cell line. We show that the cell line, designated A4, proliferated in response to IL-4 in a dose-dependent manner. The A4 cells also proliferated in response to their own B cell derived growth factor (B. BCGF), suggesting autocrine-mediated growth. The ability of IL-4 to induce proliferation of the A4 cell line was dependent on the level of autocrine growth. At low cell density, IL-4 induced marked dose-dependent proliferation. However, as A4 cell density increased, the ability of IL-4 to induce proliferation was diminished. The possibility that IL-4 may be mediating the autocrine growth of A4 cells was ruled out, because A4 cell-derived BCGF failed to induce CD23/low affinity receptors for the Fc region of IgE on activated tonsillar B cells and anti-IL-4 antibody did not block B. BCGF activity. We found that IL-4 stimulation of A4 cells and activated tonsillar B cells is associated with enhanced production of B. BCGF. These data indicate that human IL-4 has the capacity to promote proliferation of the B cell line A4, and that the ability of IL-4 to function as BCGF is associated with enhanced autocrine growth of activated B cells.