The biological response to controlled exposures of nitrogen dioxide (NO2) was studied in 19 human subjects exposed to 0.2 ppm NO2 for 2 hr and compared to 15 control subjects exposed to filtered air for 2 hr. Seven biochemical blood parameters, including glutathione, red blood cell glutathione reductase, 2,3-diphosphoglycerate, methemoglobin, vitamin E, complement C2, and IgA were measured prior to exposure, immediately after exposure, and 22 hr following exposure. The only variable that showed significant change due to NO2 exposure was glutathione. Glutathione is known to protect the erythrocyte from oxidative stress. The increase in glutathione observed upon exposure to NO2 may be a protective reaction of the erythrocyte to meet the oxidative stress.