Human erythrocytes contained a soluble cytosolic epoxide hydrolase for stereospecific enzymatic hydration of leukotriene A4 into leukotriene B4. The enzyme was purified 1100-fold, to apparent electrophoretic homogeneity, by conventional DEAE-Sephacel fractionation followed by high performance anion exchange and chromatofocusing procedures. Its characteristics include a molecular weight of 54,000 +/- 1,000, an isoelectric point 4.9 +/- 0.2, a Km apparent from 7 to 36 microM for enzymatic hydration of leukotriene A4, and a pH optimum ranging from 7 to 8. The enzyme was partially inactivated by its initial exposure to leukotriene A4. There was slow but detectable enzymatic hydration (pmol/min/mg) of certain arachidonic acid epoxides including (+/-)-14,15-oxido-5,8-11-eicosatrienoic acid and (+/-)-11,12-oxido-5,8,14-eicosatrienoic acid, but not others, including 5,6-oxido-8,11,14-eicosatrienoic acid. Human erythrocyte epoxide hydrolase did not hydrate either styrene oxide or trans-stilbene oxide. In terms of its physical properties and substrate preference for leukotriene A4, the erythrocyte enzyme differs from previously described versions of epoxide hydrolase. Human erythrocytes represent a novel source for an extrahepatic, cytosolic epoxide hydrolase with a potential physiological role.