We recently reported that Fc mu R on NK cells is a signal transducing protein that stimulates a rapid increase in the level of cytoplasmic free calcium upon binding of IgM. This study was designed to examine signal transduction via the Fc mu R on NK cells and to characterize intracellular second messengers activated by IgM. Immunoprecipitation of IgM-bound Fc mu R by IgM-specific Ab coimmunoprecipitated the zeta- and Fc epsilon RI gamma-chains. Furthermore, engagement and clustering of Fc mu R by polyclonal IgM induced tyrosine phosphorylation of the zeta- and Fc epsilon RI gamma-chains, indicating their functional association with the Fc mu R-induced signal transduction cascade. Ligand-induced clustering of the Fc mu R also induced activity of src family kinases, Lck, Fyn, Lyn, and Src, as well as their physical interaction with the receptor. Triggering via Fc mu R also induced the activity of Syk and Zap-70, tyrosine kinases demonstrated to associate with zeta and Lck. Phospholipase C-gamma 1 and phosphatidylinositol 3-kinase were identified as substrates phosphorylated on tyrosine, as down-stream components of the signaling pathway activated in NK cells by polyclonal IgM. Although the Fc mu R on NK cells has not yet been biochemically characterized, our results suggest that the zeta- and Fc epsilon RI gamma-chains are functional subunits of this as well as other important cell surface receptors and that the Fc mu R is coupled either directly or indirectly to nonreceptor tyrosine kinases, which phosphorylate and thereby activate regulatory enzymes such as phospholipase C-gamma 1 and phosphatidylinositol 3-kinase.