Leukotriene C 4 is considered to play a major role in several important pathophysiological conditions, eg, allergy, asthma, and shock. The present investigation demonstrates the presence in human platelets of a membrane-associated enzyme catalyzing the final step in the biosynthesis of leukotriene C 4. This leukotriene C 4 synthase was shown to be distinct from previously characterized “microsomal” and soluble glutathione transferases. The latter enzymes did not contribute significantly to the leukotriene A 4 conjugating activity in platelets. As determined with leukotriene C 4 synthase of a crude membrane fraction from human platelets, the K m value was 7 μM and the V value was 0.56 nmol× min− 1× mg− 1 with leukotriene A 4 as substrate. The enzyme was 20-fold more efficient with leukotriene A 4 than with leukotriene A 5 and 30-fold more efficient than with the unphysiological derivative leukotriene A 4 methyl ester, as measured by the corresponding V K m values; 14, 15-leukotriene A 4 was not a substrate. Platelets should be a useful source for the purification and further characterization of human leukotriene C 4 synthase.