Normal levels of NADH-cytochrome b5 reductase activity in platelets, lymphocytes and granulocytes were determined. The homogenate of each cell was treated with Triton X-100 after incubation with lipoprotein lipase. The reductase was extracted very well from the cells by this treatment. Moreover, the assay of the reductase activity in the cells became accurate and reproducible by the treatment. The reductase level of each cell was also determined in cases of hereditary methemoglobinemia. It was normal in the case of the disease without mental retardation, and low with mental retardation. This latter case might be due to the deficiency of cytochrome b5 reductase in the whole tissue.