Alkaline phosphatase (APase) is a glycosylphosphatidyl-inositol (GPI)-anchored protein appearing on the membranes of mitogen-stimulated B cells after progression into S phase of the cell cycle. Maximal APase expression occurs after peak proliferation and precedes maximal immunoglobulin (Ig) secretion. While APase is clearly an activation marker for mitogen-stimulated B cells, the physiologic role of APase in B cells has not been defined. Other GPI-anchored proteins have been assigned roles in transmembrane signaling since treatment with specific monoclonal antibodies (mAbs) can modulate and/or mimic the effect of mitogens or antigens. Thus, as an initial attempt to determine whether membrane APase (mAPase) plays a role in B cell activation, rat splenic B cells were treated with anti-APase specific mAb in the presence and absence of LPS plus dextran sulfate, known B cell mitogens. Anti-APase mAb alone did not induce proliferation or modulate mitogen-induced proliferation as measured by [3H]thymidine uptake and viable cell recoveries. However, the mAb augmented IgM secretion when used in a soluble form or cross-linked with anti-Ig. Both soluble and immobilized anti-APase mAb decreased the expression of APase activity by mitogen-stimulated B cells. Based upon these results we propose: (1) that transmembrane signaling may occur through mAPase as described for other GPI-anchored proteins such as Thy-1, CD55, CD59, CD24, CD73, Fc gamma III, Qa-2, Ly-6A/E and LFA-3, and (2) this signaling may be regulated by changes in protein phosphorylation caused by modulation of cellular phosphatases, specifically APase.