The role of human neutrophil cathepsin G (Cat G) on Eimeria tenella sporozoites was studied in vitro. Sporozoites were incubated for 2 hr at 37 C in PO₄ buffer, 0.9% NaCl (PBS), pH 7.6 in the presence of Cat G (50 μg/ml), diisopropyl fluorophosphate-inhibited Cat G (DFP-Cat G) (50 μg/ml) or PBS alone, prior to being inoculated into embryonated eggs. As judged by oocyst production on day 7 postinoculation, embryo mortality and the hemorrhage scores, both Cat G and DFP-Cat G demonstrated anticoccidial activity; greater activity was obtained with the DFP-Cat G. Sporozoites were exposed also to increasing concentrations of native and trypsin-digested DFP-Cat G (0-100 μg/ml) under the same conditions. Significant protection (37% and 49% for native and digested DFP-Cat G, respectively) was obtained with a low concentration (5 μ/ml), and higher concentrations resulted in 70% and 84% protection, respectively. The primary bactericidal domain of Cat G, the HPQYNQR peptide, at 3 concentrations (25, 50, and 100 μg/ml), reduced the oocyst production by 46%, 16%, and 15%, respectively. The anticoccidial activity of Cat G may involve a peptide fragment different from the antimicrobial domain of the enzyme.