Because of the discrepancy between the capacity of platelets to synthesize thromboxane B2 ex vivo and the actual synthetic rate in vivo, measurement of thromboxane B2 in plasma is highly influenced by sampling-related artifacts. We have developed and validated a radioimmunoassay for a major enzymatic derivative of thromboxane B2 with an extended plasma half-life, i.e., 11-dehydrothromboxane B2. The binding of the tracer is displaced by as low as 1 pg/ml of the homologous ligand, with a high degree of specificity for the open ring structure as well as for the omega side-chain. This method can detect changes in the plasma concentration and urinary excretion of 11-dehydrothromboxane B2 associated with stimulated short-term increases of thromboxane B2 secretion in the human circulation.