T he present study demonstrates the graded effect of in vitro corticosteroids (CSs) on the different phases of B cell activation, proliferation, and differentiation. Early events such as activation and proliferation of high-dose anti-mu or Staphylococcus aureus-stimulated B cells are profoundly suppressed by the presence of in vitro CSs. The suppressed proliferative response may be mediated by a direct effect on B cells and/or modulation of accessory cell function. Later events in the B cell cycle such as the proliferative response to B cell growth factor after either in vivo or in vitro activation are less sensitive to the suppressive effects of in vitro CSs. The final events in the B cell cycle; namely, the differentiation to the immunoglobulin-producing state, is not suppressed by in vitro CSs. Indeed, depending on the systems employed, there is either no effect or enhancement of immunoglobulin secretion by the presence of in vitro CSs. The graded effect of in vitro CSs on the discrete phases of the B cell activation, proliferation, and differentiation cycle provide new insights into the complex nature of CS-induced modulation of human B cell responses.