The effect of linoleic acid and of aspirin on platelet aggregation has been measured in six healthy volunteers with a new platelet aggregometer (Wellcome) designed to be used with whole blood. The subjects were given a controlled diet for 6 weeks during which their platelet aggregation, platelet fatty acid composition, dilute blood clot lysis time, bleeding time and serum cholesterol and triglycerides were measured. A basal diet typical of that normally eaten in the UK was fed for 3 weeks, then for a further 2 weeks 60 ml/d of safflower seed oil was added to the diet. Finally there was a further week on the basal diet and on the last day the subjects each took 900 mg of aspirin. The effects of the safflower seed oil was to increase platelet linoleic acid (C18:2 omega 6) content from 5.53 +/- 0.52 micrograms to 10.1 +/- 0.92 micrograms/100 micrograms total fatty acids (P less than 0.001), to decrease platelet aggregation to ADP, and to decreased serum cholesterol. Fibrinolysis and bleeding times were unaltered. Aspirin decreased platelet aggregation, prolonged bleeding time and increased platelet arachidonic acid (C20:4 omega 6) from 24.7 +/- 0.38 micrograms to 25.8 +/- 0.61 micrograms/100 micrograms total fatty acids (P less than 0.01). The Wellcome whole blood aggregometer is a sensitive test of platelet function and using it linoleic acid has been shown to reduce aggregation in conjunction with an increase in polyunsaturated fatty acid content of the platelet membrane.