Two first cases of prenatal diagnosis of generalized cytochrome b5 reductase deficiency are presented. In each family, there was an index case with a clinical pattern of congenital methemoglobinemia associated with severe mental retardation (type II). The foetal cells were obtained by amniocentesis at 16 weeks of pregnancy at risk. The cells were cultured, and their cytochrome b5 reductase activity was compared to control amniotic cells, and to cultured fibroblasts originating from the index case. In the first family this led to the conclusion that the foetus was normal or heterozygous. The pregnancy was continued, and the mother delivered a normal newborn with normal red-cell cytochrome b5 reductase. In the second family, the foetal cells displayed a profound decrease of cytochrome b5 reductase activity. The pregnancy was terminated and all the tissues of the aborted foetus exhibited the enzyme deficiency. It is concluded that prenatal diagnosis of the severe form of congenital methemoglobinemia can be performed without ambiguity.