Incubation of prostaglandin D2 (PGD2) with human plasma yielded a product that has been identified as 9-deoxy-9,10-didehydro-12,13-didehydro-13,14-dihydro-PGD2 (9-deoxy-delta 9, delta 12-13,14-dihydro-PGD2). The identification was based on mass spectrometry, UV spectrometry, mobilities and retention time on TLC and HPLC, and NMR. The conversion of PGD2 to this product was dependent on the incubation time and the amount of plasma added to a reaction mixture and was abolished by prior boiling. The conversion rate of PGD2 to this metabolite was 0.03 nmol/min per mg of protein of whole plasma at pH 8.0 at 37 degrees C. Similar conversion was also found by incubating PGD2 with human serum albumin added at the concentration found in plasma. These results suggest that the conversion of PGD2 to this product is catalyzed by the enzymatic action of a plasma protein, probably serum albumin. The biological activities of this compound were examined in several systems. It showed negligible activity in inhibition of human platelet aggregation and relaxation of rabbit stomach strip. On the other hand, it exhibited a three times stronger inhibitory activity (IC50, 1.8 microM) than PGD2 (IC50, 5 microM) on the growth of L-1210 cultured cells.