SUMMARY1. Studies of the effects of garlic on platelet aggregation have produced inconsistent results possibly related to variations in study design and in the garlic preparations used.2. The present study examined the effects on platelet aggregation and serum thromboxane and lyso‐platelet activating factor, of feeding garlic extract to healthy men using a placebo‐controlled, double‐blind design. The effects of the same garlic preparation on platelet aggregationin vitrowere also investigated.3. There were no significant differences in platelet aggregation with adenosine diphosphate, platelet activating factor (PAF) or collagen according to treatment group. Serum thromboxane and IysoPAF also showed no change related to garlic supplements.4.In vitroaggregation with collagen decreased linearly with increasing amounts of garlic extract, but concentrations were higher than those attainablein vivo. Gastrointestinal side effects prevented the use of higher doses of garlic which must be considered to be pharmacological as they exceed changes achievable by dietary modific