In a female newborn presenting with pronounced cyanosis in the absence of cardiopulmonary disease, the cyanosis was due to methemoglobinemia of 33% at birth and 17% at 24 hours (upper limit 0.5%) which was found to be secondary to deficiency of red blood cell cytochrome b5 reductase (EC 1.6.2.2.). Only residual activity of this enzyme was measurable, thus indicating homozygosity. Both parents were found to be heterozygous for this inherited disease. Of the six sisters and brothers of the newborn's father, five were investigated and all found to be heterozygous for the defective allele. Measurement of cytochrome b5 reductase showed both soluble and membrane bound fractions to be affected equally in the red cells of the baby's heterozygous parents.