Exposure of horse platelets to thrombin has been reported to cause nearly complete inactivation of cyclooxygenase within 30 sec. This contrasts with the observation that human platelets, depleted of their granule constituents by stimulation with thrombin, still aggregate in response to arachidonic acid, a reaction presumably mediated by thromboxane A2 (TxA2) formation. Because of this conflicting evidence, TxA2 formation was measured by radioimmunoassay in washed human platelets depleted of their alpha- and dense-storage granule constituents by prior stimulation with thrombin. These platelets aggregated in response to adenosine diphosphate (ADP), collagen, arachidonic acid, and thrombin, and formed TxA2. However, collagen- and thrombin-induced TxA2 formation by these platelets was reduced in comparison to control platelets that had not been depleted of their storage granule constituents by prior thrombin stimulation. In contrast, arachidonic acid-induced TxA2 formation was not significantly different in thrombin-depleted and control platelets. These results demonstrate that thrombin can induce degranulation of platelets without concomitant inactivation of cyclooxygenase.