An assay procedure is described in which blood cell glutathione peroxidase may be accurately measured by a direct spectrophotometric procedure. Glutathione peroxidase activity is found to be associated with a relatively stable, nondialyzable, heat-labile, intracellular component which is separable from hemoglobin, by gel filtration and ammonium sulfate precipitation. The activity appears to be dependent upon active sulfhydryl groups and is unaffected by low concentrations of azide, cyanide, or ferricyanide.