In order to assess the role of the renally formed thromboxane A2 in humans, a new radioimmunoassay was developed for the determination of the urinary excretion of thromboxane B2, the stable metabolite of thromboxane A2. Urines were extracted with ethylacetate and thromboxane B2 was separated from the other primary prostaglandins by a reversed phase high performance liquid chromatography using water, acetonitrile and acetic acid (71:29:0.1) as mobile phase with an overall recovery of 44.1% +/- 1.3 (n = 42). The radioimmunoassay, performed on the fraction containing thromboxane B2 used a sensitive (less than 1pg) and highly specific antibody (final dilution: 1/460 800) that we have raised in the rabbit. The coefficient of variation was found lower than 5% in the usable part of the standard curve and the inter-assay reproducibility was at 7.3%. The 24-hour urinary excretion of thromboxane B2 measured in 26 healthy adults established at 109 +/- 12.9 ng/24h in men and at 99.5 +/- 9.2 ng/24 h in women. Therefore, it differed from that of the other prostaglandins by lower values devoid of sex-related differences.