Platelets prevent hemorrhage by a secretory process controlled through an intracellular chain of biochemical events similar to that in all other secretory cells. Tyrosine kinase (TK) activity, which causes tyrosine phosphorylation (TP) of specific cellular proteins is temporally associated with secretion by platelets and other cells, as well as with cellular responses such as growth, contact inhibition, and malignant transformation. The role of TP in these processes is not known. We previously found that increased platelet cytoplasmic calcium accompanied secretion and TP of a 130 kD major cytoplasmic protein, whereas return of calcium to hemostatic levels in storage compartments promotes tyrosine dephosphorylation of this protein (JBC 226:16911-16916, 1991). We identified the 130 kD platelet protein as most likely being vinculin by its reaction on immunoblot and by affinity chromatography isolation with both antibodies. This year we firmly established that the protein is vinculin by immunoprecipitation with anti-vinculin and anti- phosphotyrosine, by isoelectric focusing to determine the pI, and by ruling out the presence of glycoprotein IIb/IIIa and CD31 which travel in immunoblots at approximately the same level as vinculin.