To investigate the possible involvement of tyrosine phosphorylation in the process of store-regulated Ca2+ entry, ionomycin (in the presence of EGTA) was used to deplete the intracellular Ca2+ stores of fura-2-loaded human platelets, and the effect of refilling with Ca2+, Ba2+ or Sr2+ evaluated. Depletion of the intracellular Ca2+ stores resulted in an increase in protein tyrosine phosporylation. This increase is reversed when the stores were refilled in Ca2+ or Sr2+, but not Ba2+. Refilling of the stores with Ca2+ or Sr2+, but not Ba2+, suppressed Mn2+ entry. These findings support the hypothesis that tyrosine phosphorylation plays a role in mediating store-regulated Ca2+ entry in human platelets and provides evidence for tyrosine phosphatase activity regulated by the Ca2+ content of the intracellular stores.