Leukotriene C4 (LTC4) synthase, which conjugates LTA4 and LTA4-methyl ester (LTA4-me) with glutathione (GSH) to form LTC4 and LTC4-me, respectively, has been solubilized from the microsomes of guinea pig lung and purified 91-fold in four steps to a specific activity of 692 nmol/10 min per mg protein using LTA4-me as substrate. LTC4 synthase of guinea pig lung was separated from microsomal GSH S-transferase by Sepharose CL-4B chromatography and further purified by DEAE-Sephacel chromatography, agarose-butylamine chromatography, and DEAE-3SW fast-protein liquid chromatography. It was also differentiated from the microsomal GSH S-transferase, which utilized 1-chloro-2,4-dinitrobenzene as a substrate, by its heat lability and relative resistance to inhibition by S-hexyl-GSH. The Km value of guinea pig lung LTC4 synthase for LTA4 was 3 microM and the Vmax was 108 nmol/3 min per microgram; the Km values for LTA3 and LTA5 were similar, and the Vmax values were about one-half those obtained with LTA4. The conversion of LTA4-me to LTC4-me was competitively inhibited by LTA3, LTA4, and LTA5, with respective Ki values of 1.5, 3.3, and 2.8 microM, suggesting that these substrates were recognized by a common active site. IC50 values for the inhibition of the conjugation of 20 microM LTA4-me with 5 mM GSH were 2.1 microM and 0.3 microM for LTC4 and LTC3, respectively. In contrast, LTD4 was substantially less inhibitory (IC50 greater than 40 microM), and LTE4 and LTB4 had no effect on the enzyme, indicating that the mixed type product inhibition observed was specific for sulfidopeptide leukotrienes bearing the GSH moiety.