The p85a subunit of PI3-K and Btk are two crucial components of the B-cell receptor (BCR) signalling pathway. In the present study, we showed that primary splenic B cells from p85a null and xid (Btk-deficient) mice fail to induce cyclin D2 expression and enter early G1, but not S phase of the cell cycle in response to BCR engagement. Furthermore, these Btk or p85a null B cells displayed increased cell death compared with wild type following BCR engagement. These findings are further confirmed by studies showing that specific pharmacological inhibitors of Btk (LFM-A13), PI3-K (LY294002 and Wortmannin) and PLCc (U73122) also block cyclin D2 expression and S phase entry following BCR stimulation, as well as triggering apoptosis. Collectively, these data provide evidence for the concept that the B-cell signalosome (p85a, Btk, BLNK and PLCc) is involved in regulating cyclin D2 expression in response to BCR engagement. PKC and intracellular calcium are two major downstream effectors of the B-cell signalosome and can be activated by PMA and ionomycin, respectively. In small resting (G0) B cells, costimulation with PMA and ionomycin, but not PMA or ionomycin alone, induces cyclin D2 expression and cell-cycle progression. Consistent with this, we also showed that the BCR-mediated cyclin D2 induction could be abolished by pretreatment of resting B cells with specific inhibitors of capacitative Ca 2+ entry (SK&F 96365) or PKC (Go¨6850). Our present results lead us to propose a model in which the B-cell signalosome targets cyclin D2 via the Ca 2+ and PKCdependent signalling cascades to mediate cell-cycle progression in response to BCR engagement.