A new, highly selective procedure for the determination of 11-dehydrothromboxane B2 (11-dehydro-TXB2) in human urine is described. Following the addition of [19,19,20,20-2H4]11-dehydro-TXB2 as an internal standard, samples were extracted with an affinity column of anti-11-dehydro-TXB2. Conversion of the immunoextracted 11-dehydro-TXB2 into its 1-methyl ester-11-n-propylamide-9,12,15-tris-dimethylisopropylsilyl ether derivative was followed by gas chromatography-selected-ion monitoring. The mass spectrum of the 11-dehydro-TXB2 derivative was dominated by the base peak ion of [M-C3H7]+ at m/z 698, which accounted for more than 10% of the total ion current. A typical result showed that the immunoaffinity purification procedures provided an extremely clean alternative to more conventional methods of chromatographic fractionation, and that interfering substances from the urine matrix were almost entirely eliminated during the microanalysis.