Cross-linking of surface immunoglobulin (sIg) has been shown to induce either activation or apoptosis of mature B cells presumably depending on the nature of antigens. However, the nature of antigens for induction of mature B-cell apoptosis is not yet fully understood. We cross-linked sIg of mature B cells with various amounts of either anti-Ig antibodies in the soluble form or anti-Ig coupled to erythrocytes or myeloma cells as surrogate membrane- bound antigens. Anti-Ig antibodies coupled to cell surface membrane induced rapid and extensive apoptosis of normal spleen B cells even in the absence of signalling via the Fc receptor. In contrast, soluble anti-Ig induced proliferation or apoptosis of mature B cells depending on the concentration of anti-Ig. The extent of apoptosis induced by soluble anti-Ig was limited compared to that induced by membrane-bound anti-Ig. These results suggest that mature B cells undergo apoptosis or proliferation depending on whether antigens are soluble or membrane-bound and on antigen doses.