A synthetic thrombin receptor agonist peptide (TRP) of 7 residues (SFLLRNP) was found to be as effective as thrombin for Ca2+ mobilization, and both agonists induced Ca2+ release exclusively from internal stores. Thrombin stimulated tyrosine phosphorylation of several proteins of molecular mass 40,000 Da, 42,000 Da, 70,000 Da, 120,000 Da and 130,000 Da. There was a good correlation between thrombin-induced tyrosine phosphorylation of the latter three proteins and Ca2+ mobilization.