In previous studies in animal models, Trypanosoma cruzi-induced oxidative stress and damage have sometimes been controlled by the host's anti-oxidant defence responses. The role of the anti-oxidant defence responses, such as the activities of the anti-oxidant enzymes glutathione peroxidase (GPx) and superoxide dismutase (SOD), in protection against inflammation and damage have now been investigated in humans infected with T. cruzi. The subjects were 32 asymptomatic but seropositive individuals with the indeterminate form of Chagas disease, 18 symptomatic and seropositive patients with the chronic disease, and 50 seronegative and apparently healthy controls. The inflammatory process was explored using serum concentrations of tumour necrosis factor (TNF) and NO. The serum concentrations of GPx in the patients in the indeterminate phase of infection were similar to those in the controls but much higher than those in the chronic cases (P=0.001). The serum concentrations of SOD in the patients in the indeterminate phase of infection were not only significantly higher than those in the cases of chronic Chagas disease (P=0.0004) but also significantly higher than those in the controls (P<0.001). The seropositive subjects had significantly higher serum concentrations of TNF and NO than the controls (P<0.01 for each) and the cases of chronic Chagas disease had significantly higher serum concentrations of TNF and NO than the subjects with the indeterminate form of the disease (P<0.01 for each). It therefore appears that the host's anti-oxidant defence responses (at least in terms of elevated concentrations of SOD) may inhibit inflammation during the indeterminate phase of Chagas disease.