Abstract: AIM:To investigate the effect of esomeprazole, rabeprazole, ranitidine and famotidine on antiplatelet activity of clopidogrel in patients with acute coronary syndrome (ACS). METHODS: A total of 150 hospitalized ACS patients treated with PCI were randomly divided into proton pump inhibitor (PPI) group (group A: and 40 mg/day esomeprazole in A1, n=30; 20 mg/day rabeprazole in A2, n=30); H2 receptor antagonists (H2RA) group (group B: and 300 mg/day ranitidine in B1, n=30; 40 mg/day famotidine in B2, n=30) and control group (group C: n=30). All patients received antiplatelet treatment including 300 mg/day aspirin and 300 mg/day clopidogrel as a loading dose, followed by 75 mg/day as a maintenance dose. Plasma CD62P and GP IIb/IIIa were measured by ELISA method and the platelet aggregation rate was measured by the impedance detection method. RESULTS: No significant difference was observed among groups at the reductions of CD62P, GPIIb/IIIa and PAgT before and after proton pump inhibitors or H2 receptor antagonist ingestion. When two subsets of group A or group B were compared with group C, no significant difference was seen at the reductions of CD62P, GPIIb/IIIa and PAgT. Between group A1 and A2, and between group B1 and B2, no statistical difference was found. CONCLUSION: Esomeprazole, rabeprazole, ranitidine or famotidine have no inhibitive effect on the antiplatelet activity of clopidogrel.