Abstract Overnight exposure of adult splenic B cells to anti-Ig, a surrogate for antigen/tolerogen, can result in a hyporesponsive state in terms of antibody synthesis. Since B cells treated with either intact of F(ab′) 2 fragments of anti-Ig will exit the G 0 phase of the cell cycle and enter G 1 or S, respectively, we examined which steps in B-cell activation were required for this form of hyporesponsiveness. We found that B-cell hyporesponsiveness could be induced under conditions leading to either abortive or productive B-cell cycle progression, depending on the immunogenic challenge employed. Thus, PMA + ionomycin, concanavalin A, PMA alone, or ionomycin alone induced hyporesponsiveness. Each of these reagents is able to drive B-cell exit from G 0 into G 1 and cause class II hyperexpression. We next examined the effect of cyclosporin A (CSA), a reagent that blocks anti-Ig but not by PMA-induced class II hyperexpression. Interestingly, CSA only interfered with the induction of B-cell hyporesponsiveness with anti-Ig. These results suggest that upregulation of MHC class II may be coincident with a CSA-sensitive tolerance pathway in B cells stimulated by anti-Ig. Finally, IL-4 pretreatment was found to ablate hyporesponsiveness induced by either intact anti-Ig or PMA. These results parallel the Fc-dependent induction of hyporesponsiveness reported earlier (G. Warner and D. W. Scott, J. Immunol., 146, 2185, 1991) . We propose that crosslinking of surface Ig, leading to cell cycle progression out of G 0 as well as class II hyperexpression, in the absence of a cognate T cell signal, leads to B-cell hyporesponsiveness.