Arachidonic acid (AA) is metabolized by cyclooxygenase (COX), lipoxygenase (LOX) and cytochrome P450 (CYP) in human cells to metabolites with various physiological and pathological functions. There are two forms of AA - cis and trans. The purpose of these studies was to identify, characterize and compare patterns of trans-AA and AA metabolism by human platelets, hepatic microsomes and polymorphonuclear leukocytes (PMNs), and observe the functional aspects of trans-AA on these cells. To identify and characterize the metabolites formed from trans-AA metabolism different pharmacological probes, HPLC and mass spectrometry were used. Aggregometry was used to evaluate the effects of trans-AA on platelets. In platelets incubated with 14-trans-AA, the COX pathway gave rise to all-cis-15-HETE, 14-trans-ll-HETE and 14-trans-5-'HETE; and the LOX pathway gave rise to 14-trans-12-HETE and a unique thromboxane like molecule, the structure of which was characterized. AA-induced aggregation was inhibited by 14-trans-AA with an IC50 of 7.2 μM and two unique characteristics of trans-AA were observed: (1) 14-trans-AA was not metabolized extensively by the human platelets like AA and (2) coincubation with AA stimulated basal metabolism of 14-trans-AA. 14-trans-AA is likely to interact with platelets both via metabolism and via incorporation into the platelet membrane. Profiling of 5-trans-AA metabolites generated by hepatic microsomes revealed that all four bonds participated in epoxidation and generated various epoxides, diols and hydroxyeicosatetraenoic acids (HETEs). Unlike its cis-isomer, 5,6-trans-EET was significantly more stable and resisted microsomal hydrolysis and glutathione conjugation. The microsomal metabolic profile of 5-trans-AA was different from that of AA and suggested that trans-AA are trans fatty acids that appear to be metabolically similar to xenobiotics. Experiments with PMNs suggested that 5-trans-AA was metabolized to different trans-HETEs and that these modulate cell proliferation and viability.