DURING the fetal stage of development, the human erythroid cells synthesise mainly fetal haemoglobin (α2γ2) but switch to almost exclusive formation of adult haemoglobin (α2β 2) soon after birth. Similar ontogenetic changes in the activity of the closely linked γ- and β-genes are seen in most primates and few other mammals. In goats and sheep over 95% of the haemoglobin synthesised in the fetus is of the fetal type the switch from fetal to adult haemoglobin formation starts at about 130 gestational days and is completed within a few weeks 1. Several hypotheses have been formulated to account for these striking changes in haemoglobin gene activity during ontogeny, but the factors responsible for haemoglobin switching in vivo remain unknown. We report here an experiment which shows that the fetal haematopoietic cells readily switch to adult haemoglobin formation once transplanted to an adult animal. Our findings provide evidence that the switch from fetal to adult haemoglobin formation is not restricted to the perinatal developmental stage, and raise the possibility that the haematopoietic microenvironment determines the programme of haemoglobin formation in the haematopoietic progenitor cells of the fetus or the adult animal.