A rat mAb (NIM-R5) was prepared against a 42-kD B cell activation Ag (p42). The expression of p42 is increased upon activation. NIM-R5 induces an increase of intracellular Ca2+, due to influx from the exterior milieu via calcium channels. This stimulation does not prejudice further stimulation with anti-Ig, and thus p42 constitutes an activation signal independent of membrane Ig. The antibody induces increased expression of class II molecules on resting B lymphocytes and prepares the cells for "spreading" when interacted with immobilized anti-class II antibody. The antibody alone is weakly mitogenic and comitogenic with IL-4 on resting B cells. Of particular interest, NIM-R5 induces proliferation and rescue from apoptosis in B cells activated in vitro. In conclusion, NIM-R5 induces an Ig-independent activation and proliferation of resting and activated B cells. This antibody does not recognize other known B cell activation Ag such as CD23, CD40, or CD72. We therefore propose that the p42 Ag is a glycoprotein with an important role in the regulation of B lymphocyte activation and survival.