Levels of activity of beta-aminolaevulinate synthetase, delta-aminolaevulinate dehydratase, porphyrinogen synthetase and haem synthetase in circulating reticulocytes of pre-natal FL4/Re +/+ and congenitally anaemic FL1/Re f/f mice have been determined. The activities of delta-aminolaevulinate synthetase and delta-aminolaevulinate dehydratase were found to be decreased in proportion to the hypochromia observed in mature liver-derived erythrocytes in neonatal f/f mice, but activities of enzymes later in the haem synthetic pathway were relatively undisturbed. No significant differences were found in levels of haem synthetic enzymes in foetal livers of +/+ and f/f mice. These results indicate that the severe anaemia of late prenatal and neonatal f/f mice is due both to reduction in haem synthesis expressed at the reticulocyte stage of erythroid differentiation and to restricted production of erythroid progenitor cells. Retarded foetal growth and skeletal abnormalities, both characteristic of the pre-natal expression of the f/f gene complement, can also be related to reduced levels of haem synthesis, but the abnormal distribution of pigment cells seen in f/f animals appears to be a secondary effect of reduced tissue oxygenation resulting from pre-natal anaemia.