Activation of blood platelets and their subsequent aggregation results from the interactions of several complex metabolic pathways. Considered to be of critical importance are the platelet lipids. Subsequent to platelet activation, several membrane lipids undergo hydrolysis and the free fatty acids are metabolized to prostanoids which mediate platelet function in response to vascular injury. It is conceivable then, that differences in platelet membrane fatty acid content could result in significant differences in platelet responses to aggregatory stimuli, especially between species. The objective of this study was to identify specific differences in fatty acid content between human and killer whale platelets. Blood was collected, washed platelets were prepared, and platelet fatty acids were extracted. Methyl esters of the extracted fatty acids were analyzed by gas chromatography and reported as relative concentrations. Analysis of the data revealed significant differences between the two species for several relevant fatty acids, i.e. 16:0 (P < 0.05), and 18:0, 18:1, 18:2, and 20:4 (P < 0.001). The differences in platelet fatty acid composition and concentration may explain at least some of the differences in platelet function which have previously been identified between these species.