Ligation of the B cell antigen receptor (BCR) on B lymphocytes has divergent effects including anergy, apoptosis, proliferation or differentiation depending on the developmental state of the cell. Immature B cells are sensitive to the process of negative selection enabling the removal of self-reactive B lymphocytes. Consequently, ligation of the BCR on immature B cells induces growth arrest and apoptosis. However, co-ligation of CD40 can rescue immature B cells and enable them to survive, mature and proliferate. The murine B cell lymphoma cell line WEHI-231 is widely used as a model of clonal deletion of immature B lymphocytes. This investigation has explored the signalling mechanisms used by the BCR and CD40 to differentially regulate the survival and proliferation of WEHI-231 immature B cells. In particular, the role of extracellular signal regulated kinase-mitogen-activated protein kinase (Erk- MAPK) in controlling these processes was examined. The activity of Erk-MAPK in WEHI-231 cells was investigated by Western blotting. WEHI-231 cells were shown to undergo a transient activation (≤2 hr) of Erk that was associated with the induction of the BCR-driven mitochondrial death pathway. However, a second, more sustained (12-48 hr) Erk signal was also identified and this appears to be critical for the proliferation of WEHI-231 cells. This sustained Erk signal was suppressed by ligation of the BCR and this seems to be mediated by the induction of MAPK phosphatases rather than a downregulation of upstream activators of Erk. However, co-ligation of CD40 restored sustained and cyclic activation of Erk to WEHI-231 cells and thus enabled them to undergo cell cycle progression and proliferation, as shown by measuring the uptake of [3H] thymidine. A dual role for Erk MAPK in BCR-driven apoptosis and CD40-mediated rescue of WEHI-231 cells was thus identified. The mechanisms used to regulate the activity of Erk-MAPK in WEHI-231 cells were further examined using WEHI-231 cells that were transfected with constitutively active forms of Ras. These studies showed that Ras is an important upstream activator of Erk in WEHI-231 cells and it can induce Erk1/2 activation via Raf-1 and/or phosphatidylinositol-3 kinase (PI-3-K) pathways. The induction of sustained and cyclic Erk signals and hence proliferation of WEHI-231 cells therefore requires active Ras and stimulation of its downstream effectors.