The cholesterol oxidation products (oxysterols) cholest-3,5-diene-7-one, cholestan-5 alpha, 6 alpha-epoxy-3 beta-ol (cholesterol 5 alpha-epoxide), cholestan-5 beta, 6 beta-epoxy-3 beta-ol (cholesterol 5 beta-epoxide), cholest-5-ene-3 beta-ol-7-one (7-ketocholesterol), cholest-5-ene-3 beta, 7 alpha-diol (7 alpha-hydroxycholesterol), cholestan-3 beta, 5 alpha, 6 beta-triol (cholestane triol), and cholest-5-ene-3 beta, 26-diol (27-hydroxycholesterol) potentiated platelet aggregation and increased thromboxane A2 formation in platelets challenged with thrombin, ADP or collagen. These effects were observed at oxysterol concentrations in the range 5-100 microM. Cholesterol 5 beta-epoxide and 7-ketocholesterol increased the mobilization of 3H-arachidonic acid from prelabelled platelet phospholipids in response to thrombin and collagen.