Bone marrow-derived mouse mast cells contain phospholipase C-gamma 1 (PLC-gamma 1), which is phosphorylated at tyrosine residues upon cross-linking of cell-bound IgE antibodies with multivalent antigen. It was found that immune complexes formed from digitonin lysates of the mast cells by monoclonal anti-PLC-gamma 1 antibodies contained protein-tyrosine kinase (PTK), which phosphorylated PLC-gamma 1 in vitro. The tyrosine kinase activity coprecipitated with PLC-gamma 1-anti-PLC-gamma 1 complexes markedly increased when the cell lysates were obtained immediately after antigen challenge. The results indicate that PTK is associated with PLC-gamma 1 in the mast cells and that the kinase is activated upon cross-linking of Fc epsilon RI. Neither beta nor gamma subunit of Fc epsilon RI nor src family PTK was coprecipitated with the PLC-gamma 1-anti-PLC-gamma 1 complexes. In situ denaturation/renaturation experiments, which detect autophosphorylated kinases, indicated that the PTK associated with PLC-gamma 1 was a 44-kDa protein.