In this report we have examined the effects of the protein tyrosine phosphatase inhibitor phenylarsine oxide (PAO) on receptor-mediated platelet shape change, secretion and aggregation. PAO was found to inhibit platelet aggregation induced by collagen, thrombin, ADP and epinephrine at IC50 values of 0.35 mumol/l, 2.5 mumol/l, 0.2 mumol/l and 0.3 mumol/l, respectively. Agonist-induced secretion of ATP was inhibited at similar or lower concentrations of PAO. The specificity of the interaction of PAO with platelet proteins was demonstrated by the ability of the disulfhydryl compound 2,3-dimercaptopropanol, which abstracts PAO from proteins to form a stable cyclic adduct, to reverse PAO inhibition of both agonist-induced platelet secretion and aggregation. Dimercaptopropanesulphonic acid, a membrane-impermeable analogue of dimercaptopropanol, did not reverse inhibition of collagen-induced shape change or aggregation by PAO, thereby demonstrating that PAO acted intracellularly. PAO inhibited collagen-induced shape change and internal Ca2+ mobilization but had no effect on these two phenomena when induced by thrombin or ADP. PAO was also unable to prevent arachidonic acid-induced shape change, indicating that PAO acts at a site prior to the phospholipase A2-mediated release of arachidonic acid to inhibit collagen-induced shape change. PAO induced the accumulation of a number of phosphotyrosine-containing proteins and inhibited the collagen-induced phosphorylation of a 40 kD protein. The potency and agonist-specific effects of PAO on platelet activation suggest that this inhibitor will be of value in elucidation of signal transduction pathways involved in receptor-mediated platelet function.