Leukotriene A4 hydrolase, a soluble enzyme catalyzing hydrolysis of the allylic epoxide leukotriene A4 to the dihydroxy acid leukotriene B4, was purified to apparent homogeneity from human leukocytes. The enzymatic reaction obeyed Michaelis-Menten saturation kinetics with respect to varying concentrations of leukotriene A4. An apparent KM value ranging between 20 and 30 microM was deduced from Eadie-Hofstee plots. Physical properties including molecular weight (68,000-70,000), amino acid composition, and aminoterminal sequence were determined. It was indicated that leukotriene A4 hydrolase is a monomeric protein, distinct from previously described epoxide hydrolases in liver.