Under conditions of low shear and in the absence of laminar flow the addition of acetylsalicylic acid (ASA), but not of sodium salicylate, to human platelet-rich plasma (PRP) in vitro significantly inhibited platelet adherence to collagen (p < 0.001). Inhibition was of similar degree (17%) at ASA concentrations of 10 and 1,000 μM, but was less (10% inhibition) in PRP with erythrocytes added to 40% hematocrit. The ingestion of 600 mg ASA had negligible effect on platelet adherence in whole blood or in PRP. There was no correlation between the effects of ASA on adherence and on platelet aggregation in vitro or ex vivo. It is concluded that the effect of ASA on platelet adherence is unlikely to be important in vivo, except perhaps in vascular sites where flow is non-laminar and where shear rates and/or the physical forces on platelets are low