We have studied the relationship between phagocytosis and cytotoxicity of human polymorphonuclear leucocytes (PMN) to sensitized Trypanosoma cruzi. Assays were done simultaneously using [3H]-uridine labelled epimastigotes as target cells. Phagocytosis was evaluated by the uptake and cytotoxicity by the release of parasite associated [3H]-uridine. Both reactions reached maximum levels at the same effector- to target-cell ratio and antibody concentration. Uptake of epimastigotes by PMN was highest at 30 min and intracellular disruption and release of parasite debris took place later. In conditions that precluded repeated uptake of sensitized radiolabelled T. cruzi, the release profile of [3H]-uridine from PMN that contained intracellular parasites was similar to that of the standard cytotoxic assay. However, as the ingestion phase was separated from the release step, no lag in the onset of the reaction was observed. Although we cannot rule out extracellular killing, the results of this study demonstrate that the bulk of damaged T. cruzi epimastigotes had been previously internalized by the PMN.