Trypomastigotes of Trypanosoma dionisii, a stercorarian trypanosome from bats, are effectively killed by neutrophils from human peripheral blood but are less sensitive to the cytotoxic action of human monocytes. The mechanism of killing appears to involve peroxidase and hydrogen peroxide. Trypomastigotes are as effective as epimastigotes in inducing the formation of hydrogen peroxide by effector cells. They are, however, less sensitive than epimastigotes to the cytotoxic effect of peroxidase and hydrogen peroxide. They are therefore susceptible to the high concentrations of peroxidase found in the phagosome of the neutrophil, but resist the lower levels encountered in monocytes.