Hydroxytyrosol acetate (HT-AC) is a polyphenol present in virgin olive oil (VOO) at a proportion similar to hydroxytyrosol (HT) (160–479 μmol/kg oil). The present study was designed to measure thein vitroplatelet antiaggregating activity of HT-AC in human whole blood, and compare this effect with that of HT and acetylsalicylic acid (ASA). The experiments were designed according to the standard procedure to investigate the activity of ASA. HT-AC and HT inhibited platelet aggregation induced by ADP, collagen or arachidonic acid in both whole blood and platelet-rich plasma (PRP). ASA and HT-AC had a greater effect in whole blood than in PRP when ADP or collagen was used as inducer. ASA and HT-AC had a greater effect in PRP+leucocytes than in PRP alone. All three compounds inhibited platelet thromboxane B₂and leucocyte 6-keto-prostaglandin F_1α(6-keto-PF_1α) production. The thromboxane/6-keto-PGF_1αinhibition ratio (as an indirect index of the prostanoid equilibrium) was 10·8 (se1) for HT-AC, 1·0 (se0·1) for HT and 3·3 (se0·2) for ASA. All three compounds stimulated nitric oxide production, although HT was a weaker effect. In our experiments only concentrations higher than 500 μm(HT) or 1 mm(HT-AC and ASA) inhibited 3-nitrotyrosine production. All three compounds inhibited the production of TNFα by leucocytes, with no significant differences between them. In quantitative terms HT-AC showed a greater antiplatelet aggregating activity than HT and a similar activity to that of ASA. This effect involved a decrease in platelet thromboxane synthesis and an increase in leucocyte nitric oxide production.