A radioimmunoassay was developed for 11-dehydro-TXB2, a prominent metabolite of TXB2 in blood and urine of several species. In order to reliably assay 11-dehydro-TXB2, its chemical stability as well as its chromatographic properties were first examined. Since dehydrogenation at C-11 converts the thromboxane ring into the delta-lactone form of a dicarboxylic acid, which can also occur in an open form, the analysis of 11-dehydro-TXB2 may be somewhat complicated. In some chromatographic systems, the compound thus migrated with pronounced tailing, and during extraction using the common Sep-Pak procedure the two forms were partially separated. The lactone as well as the open form could be conclusively identified using mass spectrometry. The equilibrium between the two forms of 11-dehydro-TXB2 was studied in buffers of different pH and in plasma. Higher pH favoured hydrolysis into the acyclic structure. The lactonization and hydrolysis processes were also shown to be time and temperature dependent. Two different antiplasms, raised in rabbits against conjugates of 11-dehydro-TXB2 with bovine serum albumin, displayed somewhat different properties in their recognition of the two forms of 11-dehydro-TXB2. A radioimmunoassay employing these antibodies was developed. The labeled antigen was prepared by incubation of 3H-TXB2 with rabbit lung supernatant. The limit of detection was 1.5 pg. For validation of the assay, analysis of blood and urinary samples, obtained after injection of TXB2 to a human volunteer, was done. The values obtained were compatible with previous isotope studies. Results from an inhibition experiment with rabbit lung incubated in the presence or absence of indomethacin further supported the identity of the assayed substance.