Individuals deficient in erythrocytic glucose-6-phosphate dehydrogenase (G6PD) show about a 2-fold increase in sensitivity toward dapsone-induced hemolytic anemia. Rat studies have shown that the hemolytic activity of dapsone resides in its N-hydroxy metabolites; exposure of rat red cells to N-hydroxy-dapsone in vitro followed by readministration to isologous rats results in premature splenic sequestration of the damaged cells. This study examines the ability of the steroid, epiandrosterone, to inhibit rat red cell G6PD and the effect of such inhibition on the susceptibility of rat red cells to N-hydroxydapsone hemolytic activity. Epiandrosterone was found to inhibit rat red cell G6PD uncompetitively and to suppress red cell hexose monophosphate shunt activity by more than 95%. Epiandrosterone suppression of rat red cell G6PD activity resulted in about a 2-fold increase in sensitivity of the rat cells to N-hydroxydapsone hemolytic activity, and a modest but significant increase in depletion of red cell glutathione. In contrast, suppression of rat red cell catalase activity by aminotriazole had no effect on the hemotoxicity of N-hydroxydapsone. Epiandrosterone appears to be a useful tool to explore the mechanism by which G6PD deficiency enhances susceptibility to hemolytic drugs.