We have studied the expression of the novel anti-apoptotic protein bcl-x during mouse B cell differentiation and activation. We find that bcl-x is expressed throughout all stages of B cell differentiation in the bone marrow, and is only down-regulated in mature (sIgD+) B cells. Immature peripheral B cells express low levels of bcl-x even in adult animals, whereas mature resting B cells do not. Mature B cells re-express the protein following activation, achieving maximal levels after 36-48 h. The highest levels of bcl-x are observed with potent mitogenic stimuli (such as anti-CD40 + anti-Ig): B cells first express bcl-x in the G1 phase of the cell cycle and contain maximal levels in S phase. In addition, B cells from CBA/N mice, which do not proliferate when stimulated with anti-Ig, anti-CD40 or both, exhibited only low levels of the protein following culture with these stimuli. To investigate the functional significance of bcl-x in activated B cells, we tested their sensitivity to apoptosis induced by the Ca2+ ATPase inhibitor thapsigargin: B cell blasts activated with anti-CD40 and anti-Ig were resistant to this agent. The available data therefore suggest that bcl-x fulfils two roles in B cells: it promotes survival of immature B cells (which lack bcl-2) and secondly, it apparently plays an additional role in protecting activated mature B cells (perhaps those in germinal centers) from apoptotic stimuli.