Many parasites--including the causative agents of malaria, Chagas' disease and schistosomiasis--are more susceptible to reactive oxygen species (ROS) than their hosts are. This is manifested by one or more of the following criteria: 1. Susceptibility of the parasite to ROS in vitro; 2. macrophage-based defense mechanisms against the parasite in vivo; 3. successful therapy using agents which lead to oxidative stress; 4. selection advantage (with respect to parasite infections) of human populations whose antioxidant capacity is impaired by a gene defect or by strong oxidants in their staple food. Our laboratory is involved in developing inhibitors against antioxidant enzymes thus mimicking natural experiments. Since glutathione reductase is a protein of known atomic structure the methods of drug design by receptor fit (DDRF) can be applied for this enzyme. Another promising target enzyme is trypanothione reductase which was found so far only in trypanosomatids, and specifically, not in their hosts. Consequently the trypanothione pathway may be a general target in the design of drugs against diseases caused by trypanosomes and leishmanias.