Human platelet activation is associated with, and regulated by, the phosphorylation of a number of proteins. Recently, attention has been focused on tyrosine phosphorylation of proteins and their function in platelet activation. Here vanadate, an inhibitor of tyrosine phosphohydrolase, was used to examine the role that tyrosine phosphorylation plays in platelet activation. Vanadate (7.5 to 100 mumol/L) stimulated the dose-dependent aggregation of saponin- permeabilized, but not intact, platelets. Electron-microscopic studies indicated small degranulated aggregates. Vanadate-induced aggregation was inhibited by pretreatment with prostacyclin (1 to 10 nmol/L), genistein (1 to 10 micrograms/mL), aspirin (100 mumol/L), or BW755C (80 mumol/L). Aggregation was associated with the aspirin-sensitive formation of [32P]phosphatidic acid and the phosphorylation of platelet proteins, notably pleckstrin and myosin light chain. Immunoblotting studies indicated that vanadate caused the tyrosine phosphorylation of proteins of approximate molecular weights 26, 29, 32, 40, 42, 80, and 90 Kd. Preincubation with BW755C abolished the phosphorylation of the 26-, 29-, 32-, 40-, and 42-Kd proteins but not the 80- and 90-Kd proteins. Vanadate stimulated the release of [3H]-arachidonic acid that was not affected by pretreatment with BW755C. The subsequent conversion of [3H]-arachidonic acid to [3H]-thromboxane A2 was significantly inhibited. These findings show that vanadate stimulates platelets by promoting arachidonic acid release from phospholipids. Tyrosine phosphorylation, potentially of the 80- or 90-Kd proteins, may regulate a platelet phospholipase A2. The release arachidonic acid was converted to thromboxane A2 that produced secondary effects such as phospholipase C activation, protein phosphorylation, and aggregation, and was associated with the tyrosine phosphorylation of the 26-, 29-, 32-, 40-, and 42-Kd proteins.