The efficiency of human granulocytes from patients infected with Schistosoma mansoni in killing either 2-hour or 24-hour schistosomula in the presence of complement was 43.0 ± 5.0% and 22.7 ± 1.8%, respectively. Granulocyte, antibody, and complement killed 61.1 ± 1.7% of 2-hour schistosomula and 13.9 ± 1.9% of schistosomula previously incubated for 24 hours in complex medium. The resistance of 24-hour schistosomula was less marked when they were incubated in defined medium (20.0 ± 1.5%). When the 24-hour incubation was performed in the presence of puromycin, however, the death of schistosomula remained high: 51.9 ± 5.7% with cells plus complement or 48.7 ± 4.8% with cells, antibody, and complement. The increase in eosinophil percentage in granulocyte suspensions did not enhance their ability to kill 24-hour schistosomula, but it was associated with a slight increase in the degree of damage inflicted on the 2-hour schistosomula.