AbstractEvidence is presented that cardiolipin, a naturally occurring phospholipid, inhibits the aggregatory effect of platelet‐activating factor (paf) on rabbit plateletsin vitro. Bovine heart cardiolipin was shown to inhibit the aggregation of washed rabbit platelets induced by 1×10−10 M and 2×10−10 M paf with IC50 values (doses for half‐maximal inhibition) of 8.4±0.8×10−7 M and 2.6±0.6×10−6 M, respectively. Phosphonocardiolipin was also able to inhibit platelet aggregation induced by 1× 10−10 M paf with an IC50 value of 3±1×10−7M. Both compounds, in concentrations up to 1×10−5 M, were unable to aggregate washed rabbit platelets and failed to inhibit the aggregation induced by 0.9 and 1.8 μM adenosine diphosphate or 0.2–1.0 μM arrchidonic acid. By contrast, the acetylated derivative of cardiolipin exerted an aggregatory effect on aspirin‐treated rabbit platelets in the presence of creatine phosphate/creatine phosphokinase. This aggregation was inhibited by the specific paf antagonists BN 52021 and WEB 2086. Also, platelets treated with acetyl‐cardiolipin were insensitive to the aggregatory effect of paf. Phosphatidic acid, phosphatidylglycerol,bis(dipalmitoylglycero)phosphate and their phosphono analogues were totally inactive. Similar data were obtained when platelet‐rich plasma was used instead of washed rabbit platelets. Our results support the hypothesis that the effect of cardiolipin is mediated through specific paf receptors that act on the rabbit platelet membrane.