High levels of antimalarial drugs have been shown to be inhibitory to the oxidative metabolism of human neutrophils in vitro. In this study, chemiluminescence of, and chloroquine concentrations in, neutrophils from volunteers treated with prophylactic or therapeutic malaria regimens were assessed. Although detectable chloroquine levels were found in neutrophils, their chemiluminescence was not reduced at prophylactic or at therapeutic drug levels, in comparison with controls without chloroquine treatment. At these clinically achievable concentrations, chloroquine was not found to interfere with the oxidative metabolism of neutrophils.