Following the observation of two fraternal patients without neurologic symptoms, but with hereditary methemoglobinemia due to cytochrome b5 reductase deficiency in erythrocytic and nonerythrocytic cells, a familial study of their paternal and maternal relatives was undertaken. Ferrihemoglobin reductase activities in erythrocytes from the two patients were found to be impaired, and cytochrome b5 reductase activities in platelets and leukocytes were essentially absent. Any deficiencies of the enzyme activities seemed not to be found in nonhematopoietic cells. The enzyme activities in blood cells derived from the parents and some of their paternal and maternal family members showed levels intermediate between those of the patients and those of the normal control, which seemed to be heterozygous. The present cases did not belong to either the classic erythrocytic or the generalized type, and their enzyme deficiency was found rather to be restricted to their blood cells and not associated with neurologic and mental disorders. A necessity of diagnosis by tissues other than blood cells is discussed for a severe form of generalized-type hereditary methemoglobinemia with associated neurologic and mental disorders.