We have measured Ca2+ mobilization in a panel of B lineage cell lines after stimulation with anti-Ig to assess whether membrane Ig transduces a functional signal in cells that are representative of immature, mature, or terminally differentiated stages. For these studies, three transfected cell lines which express the same IgM molecule (300-19μmλ36/8, K46-17μmλ, and J558Lμmλ3) as well as two lines expressing an identical IgD molecule (K46δm2.6 and J558Lδm8.8) were used. Cross-linking of membrane Ig on IgM+ or IgD+ lymphomas (K46-17μmλ or K46δm2.6) resulted in a Ca2+ mobilization response that is similar to that seen in mature, resting B cells. Both intracellular release and extracellular influx of Ca2+ were observed. In contrast, ligation of membrane Ig on an IgM+ pre-B cell line (300-19μmλ36/8) induced extracellular influx of Ca2+ but no detectable intracellular release. Finally, cross-linking of membrane Ig on IgM+ or IgD+ plasmacytomas (J558Lμm3 of J558Lδm8.8) or an IgD+ B cell hybridoma (B1.8.δ1) expressing an endogenous Ig gene, did not result in a detectable Ca2+ mobilization response. Importantly, stimultion of cells with the GTP-binding protein activator, aluminum fluoride, resulted in a comparable Ca2+ mobilization respopnse in all cell lines. In view of the fact that aluminum fluoride induced a Ca2+ response in the terminally differentiated B cell lines, J558Lμm3, J558Lδm8.8, and B1.8.δ1, it is likely that there is an alteration in the signal transduction cascade at some point proximal to GTP binding protein activation. This finding suggests that differentiation of the B cell is accompanied by the loss or alteration of one or more components that couple membrane Ig to subsequent signal transduction elements. Finally, it has previously been demonstrated that the IgM+ cell lines described above, express the recently described membrane Ig-associated protein, B34. Thus, it is apparent based on the fact that the J558Lμm3 cell line does not mobilize Ca2+ after stimulation with anti-Ig, that coexpression of B34 in association with membrane Ig does not constitute a functional receptor complex capable of activating GTP-binding proteins that in turn regulate Ca2+ mobilization.