Fish oil diets, rich in n-3 polyunsaturated fatty acids, are considered to have an antithrombotic effect. Both platelets and endothelial cells play a crucial role in the regulation of thrombosis and haemostasis. There is substantial evidence that, in these cells, fish oil-derived polyunsaturated fatty acids can replace the n-6 polyunsaturated fatty acid, arachidonic acid, in the membrane phospholipids and can modify those cellular reactions in which the latter fatty acid participates. However, it now appears that dietary n-3 polyunsaturated fatty acids are less potent than, for example, aspirin in modifying the activation properties of these cell systems. This suggests the possible involvement of additional cells or factors in mediating the antithrombotic potential of fish oil fatty acids.