Using a radioimmunoassay for rabbit-serum albumin, platelet-activating-factor-induced serum-albumin release by platelets was monitored under non-aggregating conditions. The four main results from this study are as follows. The ECS0 of the release was o f t h e same order of magnitude as the aggregation EC,, in the presence of calcium. The rclease took place within 2 min and was inhibited by BN 52021, which is a very specific inhibitor of the platelet-activating-factor-aggregating effect. Serum-albumin release was much greater than serotonin release.Many metabolic events occur after agonist binding to platelet membranes [I]. Besides secondary-messenger appearance in the cytosol, a fast and ATP-dependent actin polymerization causes the well known shape change of platelets. Platelet aggregation then occurs just after, or during, the release of the internal content of dense and a-granules, initiating the chain reaction activating new platelets.The components and properties of membranes are also modified. Platelet aggregation is provoked by the appearance on the cell surface of new proteins [2] and/or new phospholipids [3]. These factors come from plasma (coagulation factors [4], fibrinogen [5]) or from within the platelets (fibrinogen [6], different coagulation factors [7,8], phosphatidylserine [9]) depending on the organism. The present study demonstrates that a protein very similar to serum albumin is released from rabbit platelets under the influence of platelet activating factor (PAF-acether), with some evidence of a small contribution from cellular granules. MATERIALS A N D METHODS Radio-iodination of' rabbit-serum albuminRabbit-serum albumin (RSA, fraction V, Sigma) was radiolabeled to a specific activity of 76 Ci/mmol with "' I (New England Nuclear) by the chloramin-T method described by Greenwood et al. [lo]. The '2sI-labeled RSA (lz5I-RSA) preparation was subsequently diluted to 6 x 10' cpm/ml in NET buffer (150 mM NaC1, 5 m M EDTA, 50 mM Tris, and 0.02% sodium azide, pH 7.4) containing 1 mg/ml ovalbumin (Calbiochem-Behring ; NET-ovalbumin buffer). More than 97% of the radioactivity was acid-precipitable.