A human B cell clone (SGB3) responsive to IL 2 was established. Both recombinant IL 2 and B cell differentiation factor (BCDF) induced IgG secretion in SGB3 cells in a dose-dependent manner, but IL 2 did not affect the proliferation of SGB3 cells. FACS analysis showed that SGB3 cells expressed Tac antigen and anti-Tac antibody inhibited IL 2-induced IgG secretion without any inhibitory effect on BCDF-induced IgG secretion. These results showed that IL 2 could directly act on B cells and provide a differentiation signal through IL 2 receptors distinct from BCDF receptors. Physiologic relevance of IL 2 in the antibody response was discussed.