Stimulation of B cells from BALB/c with allogeneic lymphocytes from C57BL/6 mice resulted in a slight increase in cytosolic Ca2+ but in the absence of proliferative response. Immunization of BALB/c mice with C57BL/6 total lymphocytes resulted in an enhancement of cytosolic Ca2+ and of B cell proliferation. Phosphatidylinositol specific phospholipase C was activated immediately after allogeneic stimulation as deduced by the concomitant rise in inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. Translocation of protein kinase C from the cytosol toward the membranes paralleled the elevation in cytosolic free Ca2+. Activation of BALB/c B cells with solubilized alloantigen from the plasma membrane of C57BL/6 lymphocytes produced qualitatively the same early responses as the treatment with allogeneic cells, although quantitatively more intense. Concerning protein kinase C, an important degradation was observed in these conditions. Soluble alloantigen failed to promote B cell proliferation, being observed when cells were costimulated with a low concentrations (2 ng/ml) of phorbol 12,13-dibutyrate before alloantigen addition. Analysis of the molecular weight of the active fraction of the solubilized alloantigen revealed the presence of a 51 kDa protein that mimicked all properties of the alloantigen preparation. This molecule was also recognized in Western blot by an anticlass I mAb and by the sera of immunized animals. A putative MHC class I antigen is proposed as the nature of the active molecule, and its interaction with specific membrane Ig on the B cell is analyzed. Although the results fit with a cellular response mediated through membrane Ig, the involvement of other B cell surface molecules interacting with the alloantigens cannot be disregarded.