Unsaturated fatty acids (FA) were bound to human albumin in a molar ratio of 2 and were then added to platelet-rich plasma (PRP), increasing the total FA concentration by 1 mM. Separate FA and FA paired with di-homo-γ-linolenic acid (DHLA) and arachidon-ic acid (AA), respectively, were incubated with PRP for 5 and 90 min, and platelet aggregation and platelet factor 3 (PF3) activity were measured. DHLA inhibited adenosine-diphos-phate- and collagen-induced platelet aggregation after 5 min, an effect which was enhanced after 90 min. The effects of AA, which stimulated platelet aggregation and increased PF3 activity, were partially counteracted by DHLA after 90 min. When linoleic acid and AA were incubated with PRP in equimolar concentrations for 90 min, an inhibition of collagen-induced platelet aggregation was observed which was not observed after incubation with these FA separately. This study indicates that exogenous albumin-bound AA and DHLA may be used directly by the platelet cyclo-oxygenase pathway before significant incorporation into the platelet phosphoüpids has occurre