We investigated the possible role of tyrosine phosphorylation in the activation process of mast cells by cross-linking of cell-bound IgE antibodies. Bone marrow-derived mouse mast cells (BMMC) were sensitized with mouse IgE antiDNP mAb and then challenged with multivalent Ag DNP conjugates of human serum albumin. Analysis of phosphotyrosine-containing proteins in their lysates by SDS-PAGE and immunoblotting revealed that cross-linking of cell-bound IgE antibodies induced a marked increase in tyrosine phosphorylation of several proteins. To obtain direct evidence for activation of protein-tyrosine kinases (PTK), phosphotyrosine-containing proteins in lysates of mast cells were affinity purified, and kinase activity of the immunoprecipitates was assessed by an in vitro kinase assay. The results clearly showed activation of PTK upon cross-linking of Fc epsilon RI. Activation of PTK was not detected by the same assay when the sensitized BMMC were challenged with monovalent DNP-lysine. Treatment of sensitized BMMC with either Ca2+ ionophore or PMA failed to induce the activation of PTK. A representative IgE-independent secretagogue, thrombin, induced histamine release from BMMC but failed to induce activation of PTK. The results excluded the possibility that PTK activation is the consequence of an increase in intracellular Ca2+ or activation of protein kinase C. Addition of genistein, a PTK inhibitor, to sensitized BMMC before Ag challenge inhibited not only Ag-induced PTK activation, but also inositol 1,4,5-trisphosphate production, and histamine release in a similar dose-response relationship. Other PTK inhibitors, such as lavendustin A and tyrphostin RG50864, also inhibited the Ag-induced activation of PTK and histamine release. The results collectively suggest that activation of PTK is an early event upstream of the activation of phospholipase C, and is involved in transduction of IgE-dependent triggering signals to mediator release.