Summary. The incidences of erythroid colony forming cells (CFU_e) and granulocyte‐macrophage colony forming cells (CFU_C) have been measured in 11–18 d prenatal livers of mice of genotype f/f and nearly congenic +/+ controls. In normal fetal livers numbers of CFU_e (cells able to form colonies of 16 or more cells after 72 h in vitro) rise to a maximum on day 14 of gestation and represent c 1% of total fetal liver cells. In f/f fetal livers, peak values for numbers and proportions of CFU_e are 50% of normal. They f/f lesion does not reduce the numbers of CFU_c in fetal liver. Since this deficiency in CFU_e parallels deficiencies of similar magnitude in spleen‐colony forming units (CFU_s) and erythroblasts in the liver, and erythrocytes in the blood, of f/f fetuses it is concluded that the f/f lesion is expressed at an early stage of haemopoietic development in prenatal life. The possibility that restricted haem synthesis is the primary effect of the f/f genotype and responsible for disturbances of both haemopoietic cellular proliferation and haemoglobin synthesis is examined.