Activated leukocytes participate in immunity to infection by the parasitic blood fluke Schistosoma mansoni. They attach to the surface of schistosomes and secrete schistosomicidal substances. Cationic proteins, hydrolytic enzymes, and oxidants, produced by the leukocytes, have been implicated in the damage to the schistosomes. To examine the possible involvement of elastase in the killing of schistosomes by leukocytes, young and adult stages of S. mansoni were treated in vitro with pancreatic elastase (PE) and neutrophil elastase (NE). Schistosomula, lung-stage schistosomula (LSS), and adult worms (AW) have been found to be sensitive to both PE and NE. Male AW were more sensitive to PE than female AW. The enzymatic activity of elastase is essential for its toxic effect because heat-inactivation and specific elastase inhibitors prevented elastase-mediated schistosome killing. Thus, α1-antitrypsin and the chloromethyl ketone (CMK)–derived tetrapeptides Ala-Ala-Pro-Val-CMK and Ala-Ala-Pro-Ala-CMK but not Ala-Ala-Pro-Phe-CMK and Ala-Ala-Pro-Leu-CMK blocked PE caseinolytic and schistosomulicidal activities. As shown previously, schistosomes are also efficiently killed by hydrogen peroxide. LSS appear to be more resistant than AW and early-stage schistosomula to the lytic effects of hydrogen peroxide. Cotreatment experiments with both elastase and hydrogen peroxide indicated that they exert an additive toxic effect and that hydrogen peroxide sensitizes schistosomula to the toxic effect of elastase but not vice versa. These results demonstrate, for the first time, that elastases may be toxic molecules used by neutrophils, eosinophils, and macrophages to kill various developmental stages of S. mansoni.