In vitro studies examining the metabolic transformation of prostaglandin D2 (PGD2) by human liver were conducted. PGD2 was found to be converted by a NADPH-dependent enzyme in the 100,000 X g supernatant of human liver exclusively to a single more polar compound that had a mass spectrum essentially the same as that of prostaglandin F2 alpha (PGF2 alpha). However, this compound could be chromatographically separated from PGF2 alpha and failed to form a butylboronate derivative. The structure of this compound was established as 9 alpha, 11 beta-(15S)-trihydroxyprosta-(5Z, 13E)-dien-1-oic acid (9 alpha, 11 beta-PGF2) by comparison of its chromatographic and mass spectral characteristics with authentic 9 alpha, 11 beta-PGF2. This compound was found to be biologically active by demonstrating increases in blood pressure in rats in a dose-related fashion following intravenous administration. By using a mass spectrometric assay, levels of this compound in plasma and urine from a normal volunteer were 6 pg/ml and 982 ng/24 hr, respectively. In a patient with systemic mastocytosis associated with overproduction of PGD2, urinary excretion of 9 alpha, 11 beta-PGF 2 was 6634 ng/24 hr and a circulating plasma level as high as 490 ng/ml was found during a severe episode of systemic mast cell activation. 9 alpha, 11 beta-PGF2 is structurally a unique prostaglandin, is enzymatically formed, is produced in vivo in humans, and is biologically active.