The IgM Fc receptor (FCμR) is the newest FcR, and coligation of FCμR and Fas/CD95 on Jurkat cells with agonistic IgM anti-Fas mAb was shown to inhibit Fas-induced apoptosis. The ligand-binding activity of human FCμR was further examined. FcfjiR-mediated protection from apoptosis was partially blocked by addition of 104 molar excess of IgM or its soluble immune complexes, but it could be inhibited by addition of 10-fold excess of IgM anti-CD2 mAb. This suggests that FCμR binds more efficiently to the Fc portion of IgM reactive with plasma-membrane proteins than to the Fc portion of IgM in solution. The former interaction occurred in cis on the same cell surface, but not in trans between neighboring cells. This cis engagement of FCμR resulted in modulation of Ca2+ mobilization via CD2 on Jurkat cells or BCRs on blood B cells upon cross-linkage with the corresponding IgM mAbs. Several functional changes were observed with FCμR mutants: 1) significant increase in IgM ligand binding in the cytoplasmic tail-deletion mutant, 2) enhanced cap formation in FCμR upon IgM binding at 4°C with a point mutation of the transmembrane His to Phe, and 3) less protective activity of FCμR in IgM anti-Fas mAb-mediated apoptosis assays with a point mutation of the membrane-proximal Tyr to Phe. These findings show the importance of the cis engagement of FCμR and its critical role in receptor function. Hence, FCμR on B, T, and NK cells may modulate the function of surface proteins recognized by natural or immune IgM Abs on the shared membrane cell surface.