Transformation of leukotriene (LT) A4 into leukotriene C4 has been found to be carried out by human platelets in a rather efficient manner. LTC4 was characterized by a combination of high performance liquid chromatography, UV spectrophotometry, use of labeled precursor, guinea pig ileum bioassay, and enzyme immunoassay. LTA4 metabolism was found to be substrate-dependent, time-dependent, and proportional to platelet concentration even at sub- or supraphysiological levels (0.0019-1 X 10(9) platelets/ml). Neither plasma alone nor the supernatant of resting or activated platelets was found to catalyze the production of LTC4 in the presence or in the absence of reduced glutathione. These data suggest that platelets contain a glutathione S-transferase specific for LTC4 biosynthesis. The formation of LTC4 was greatly enhanced when LTA4 was incubated with platelets in the presence of albumin. Low concentrations of albumin (2-4 g/liter) stabilized LTA4 to an extent that conversion into LTC4 by the platelets could be detected after 1 h of incubation. The possible intercellular transfer of LTA4 between neutrophils and platelets was tested. The production of LTC4 by neutrophils was greatly enhanced in the presence of platelets. Furthermore, the supernatant of neutrophils stimulated with the calcium ionophore contained a short-lived acid-labile substance which was converted by the platelets into LTC4. When platelets were prelabeled with [35S]cysteine to allow intracellular synthesis of [35S]glutathione, the coincubation of both cell types challenged with the calcium ionophore resulted in the production of [35S] LTC4. These data indicate that platelets can produce large amounts of LTC4 from neutrophil-derived LTA4. They also suggest that such interactions may occur in vivo and that platelets could be an important contribution to the generation of the biologically active LTC4.