Abstract Purified human eosinophils were challenged with N-formyl-methionyl-leucyl-phenylalanine, leukotriene B 4, platelet-activating-factor, valyl-glycyl-seryl-glutamic acid, phorbol myristate acetate, zymosan, opsonized zymosan and the calcium ionophore A23187 to induce leukotriene synthesis. Reversed-phase high performance liquid chromatography analysis demonstrated the almost exclusive synthesis of leukotriene C 4 by eosinophils of 11 healthy donors after challenge with opsonized zymosan [(22±4) × 10 6 molecules LTC4/cell, mean ± SE] or the calcium ionophore A23187 [(54 ± 7) × 10 6 molecules LTC4/cell, mean ± SE]. The other agents were not capable of inducing leukotriene formation. When in addition to opsonized zymosan N-formyl-methionyl-leucyl-phenylalanine or platelet-activating factor were added a significant increase of the leukotriene C 4 synthesis by eosinophils was observed. These results suggest that eosinophils might be triggered to produce considerable amounts of the spasmogenic leukotriene C 4 in vivo by C3b- and/or IgG-mediated mechanisms e.g. phagocytosis.