To test the hypothesis that the resistance of sickle trait (AS Hgb) erythrocytes (rbcs) to malaria may be mediated by increased production of activated oxygen species, the production of superoxide anion (O2-) and hydrogen peroxide (H2O2) by AS rbcs and normal (AA Hgb) rbcs was measured under defined conditions. Formation of O2- and H2O2 was time, temperature and oxygen saturation dependent. Reproducible measurement of O2- formation required the presence of 0.2 mmol l-1 KCN to inhibit a cytochrome oxidase activity found in the cytochrome C preparation used. There was an inverse relationship between cell concentration and O2- and H2O2 formation. Use of the inhibitor of superoxide dismutase (SOD), diethyldithiocarbamic acid, increased the amount of O2- measured. When rbcs from blacks with AS Hgb and with AA Hgb were incubated under standardized conditions, significantly (P less than 0.05) more O2- was formed by AS than AA cells (24.3 v. 14.5 mmol per mol Hgb). These findings show that AS rbcs can generate more O2- than AA rbcs. The increased formation of O2- by rbcs containing AS Hgb may contribute to the resistance of AS rbcs to malarial parasitism.