Albumin is the major protein in plasma and possesses high affinity binding sites for fatty acids. Previous studies have shown that albumin interferes at various levels with the metabolism of arachidonic acid by stimulated human platelets. The aim of our study was to further characterise the effect of serum albumin on the release of arachidonic acid from platelet phospholipids and on the formation of thromboxane B2In washed prelabeled human platelets, stimulated with thrombin (0.5 U/ml), the presence of albumin in the incubation medium leads to an accumulation of [3H]AA in the extracellular space and to a reduced formation of thromboxane B2. In an albumin-free medium, the radioactivity of thromboxane B2is markedly greater, while that of arachidonate is much less. The effect of bovine serum albumin is dose-dependent (0.35%, 1.0% and 3.5%). These data suggest that arachidonic acid liberated by PLA2-activation is released to the extracellular space where it binds serum albumin and thus is no longer available for its metabolic conversion to thromboxane B2.