Human erythrocyte NADH-cytochrome b₅ reductase, or b₅ reductase, plays a major role in the reduction of methemoglobin,deficiency of which will lead to hereditary methemoglobinemia. Deterrnination of b₅ reductase activity is usually done by spectrophotometry. A new method has been developed for the qualitative and semi-quantitative detection of b₅ reductase activity, in which antibodies against b₅ reductase was dot-blotted onto nitrocellulosemembrane, and this in turn was used to capture and enrich b₅ reductase activity were visualized with the precipitable substrate MTT, or 3-(4, 5-dimethyl thiazolyl-2)-2, 5-diphenyl-tetrozolium bromide. Being straightforward and easy to follow, this method provides a new approach for the diagnosis of hereditary methemoglobinemia.