The Ag-specific interaction between cloned allospecific human Th cells and class II MHC determinants on the surface of allogeneic B cells induces a significant fraction of resting B cells to express a B cell specific activation Ag BLAST-2 (CD23). On the other hand, cross-linking of B cell surface Ig R by Ag analogues does not lead to BLAST-2 expression. By utilizing the BLAST-2 induction assay as a positive control for efficient Th-B cell interaction, we have investigated the biochemical basis of human B cell activation mediated by Ag and Th cells. Our data demonstrate that ligands for sIg R, including F(ab')2 goat anti-human IgM and Staphylococcus aureus protein A, stimulate the metabolism of B cell membrane inositol lipids as assessed by: 1) increased [3H]inositol phosphates formation in myo-[3H]inositol-labeled B cells; 2) selective incorporation of [32P]orthophosphate into phosphatidic acid and phosphatidylinositol, but not into phosphatidylethanolamine or phosphatidylcholine; and 3) rapid increase in B cell cytoplasmic ionized Ca2+ concentration ([Ca2+]i). In contrast, direct Th-B cell interaction leads to high intensity BLAST-2 expression on the B cell surface but this response is not mediated by changes in inositol lipid metabolism or [Ca2+]i. Further, Th-B cell interaction does not affect the changes in B cell inositol lipid metabolism or [Ca2+]i triggered by sIg cross-linking. Taken together, our results suggest that Ag and Th cells induce different functional B cell responses by activating distinct second messenger systems within the B cell.