H uman as well as murine granulocytes have been shown to kill the larval stages of helminth parasites; the mechanism of this cell-mediated cytotoxicity is, however, poorly understood. The present study was designed to assess the role of peroxidative processes in killing of schistosomula of Schistosoma mansoni by human granulocytes in vitro. The rate of H₂O₂ production by human neutrophils, eosinophils, and basophils was measured upon incubation with schistosomula alone or in the presence of specific antibody or complement. Opsonized parasites (antibody and/or complement) increased the rate of H₂O₂ production by neutrophils, eosinophils, and basophils by respective percentages of 500, 500, and 371. The rate of H₂O₂ release was directly related to the number of granulocytes and to the proportion of cells attached to the surface of the schistosomula. Increased hydrogen peroxide release occurred by 10 min of incubation and was demonstrable up to 16 h after addition of leukocytes to schistosomula. The primary source of this oxygen product was found to be the granulocytes adherent to the schistosomula and not those that remained unattached. Hydrogen peroxide production by neutrophils and eosinophils was quantitatively similar (schistosomula coated with antibody plus complement stimulated 5 × 10⁶ neutrophils and eosinophils to release H₂O₂ at respective rates of 0.35 and 0.40 nmol/min). Granulocyte-mediated parasite killing correlated with rate of H₂O₂ generation; both processes were inhibited by catalase. To define further the role of oxidative metabolites, neutrophils and eosinophils of two subjects with chronic granulomatous disease were used; marked reduction of granulocyte-mediated parasite mortality was observed.Peroxidase was required for H₂O₂-mediated killing. Addition of the peroxidase inhibitors azide (1 mM), cyanide (1 mM), or aminotriazole (1 cM) to neutrophilschistosomula mixtures significantly reduced parasite cytotoxicity (P < 0.01); similar reduction was observed when eosinophils were used (P < 0.01). Fixation of halide (iodide) to trichloroacetic acid-precipitable protein (2.4-6.0 nmol/h per 10⁷ neutrophils) was demonstrated in the presence of granulocytes, opsonins, and parasites; this process was completely inhibited by 1 mM azide.These data indicate that contact between the surfaces of human granulocytes and schistosomula results in release of cellular hydrogen peroxide and iodination. The generation of H₂O₂ and its interaction with peroxidase appear to be crucial in effecting in vitro granulocyte-mediated parasite cytotoxicity.