Partially hydrogenated soybean oil (PHSBO) and natural soybean oil (SBO) were obtained from a commercial source and their fatty acids were fractionated into saturates, monoenes and diene fractions. The present study compared the effect of the total, monoene and diene fatty acid fractions of PHSBO with those of the SBO on the fatty acid composition of the cell phospholipids (PL) and the prostacyclin (PGI(2)) release by endothelial cells (EC) in culture. Results showed that arachidonic acid (AA) level decreased significantly and linoleic acid (LA) significantly increased in the cells incubated with the diene fraction or the monoene fraction of PHSBO plus 18:2 at 3:1 ratio compared to the cell incubated with those fractions of SBO. These changes were attributed to the inhibition of LA conversion to AA by trans 18:1 and 18:2 isomers present in the monoene or diene fractions of PHSBO leading to a significant decrease of PGI(2) released by the cells incubated with monoene or diene fractions of PHSBO. The cells incubated with the monoene of PHSBO or SBO plus 18:2 at a 1:1 ratio showed no inhibition of LA conversion to AA and the level of AA was almost equal in their PL, but the PGI(2) released by the cells incubated with the monoene of PHSBO was significantly less than the cells incubated with the monoene of SBO. This decrease was not related to the inhibition of PGI(2) synthesizing enzymes or phospholipase (PLA(2)) activities. Our data show that trans acids in PHSBO inhibited the PGI(2) release by the cells through controlling the level of AA as substrate, either by (a) inhibiting the conversion of LA to AA or (b) by shunting the free AA released by the PLA(2) action to metabolism by another pathway leaving less AA available for PGI(2) synthesis.