A test system that allows a precise monitoring of intracellular killing of Leishmania parasites was used to measure the leishmanicidal capacity of activated macrophages from different strains of mice. Activation was obtained by exposure to dilutions of lymphokine (LK)-rich medium in the presence of ng/ml amounts of E. coli lipopolysaccharide (LPS). Highest leishmanicidal activity was displayed by macrophages from the healer mouse strain CBA/T6, whereas cells from the nonhealer strains DBA/2 and BALB/c were less effective. C3H/HeJ macrophages from a healer but LPS-unresponsive mouse strain failed to destroy leishmanias under these conditions. Experiments were then performed to determine the level of respiratory burst activity in the various macrophage populations. Hexose monophosphate shunt stimulation was higher in CBA/T6 than in BALB/c or DBA/2 macrophages, and only marginal in C3H/HeJ cells, correlating with differing leishmanicidal activities of such macrophages under the present experimental conditions. Measurements of O2- and H2O2 secretion and of chemiluminescence led to similar findings, i.e., CBA/T6 macrophages released higher amounts of oxygen metabolites than BALB/c cells activated under the same conditions, whereas C3H/HeJ cells were the least active. The results obtained by the four assays of oxidative metabolism were consistent in that endotoxin itself (in the 10 to 30 ng/ml range) stimulated the oxidative response of macrophages to a level close to that achieved by using LPS and LK together, however, only in the latter situation were parasites destroyed.