Normal adults may have two distinct erythroid precursor populations, a major one which produces only adult haemoglobin (HbA), and another which produces HbA and fetal haemoglobin (H0F) (F cells). Persistence or apparent reactivation of HbF production in adults results from differential selection of these F cells, except in those rare conditions which are due to specific deletions of D.N.A. involved in suppression of gamma-chain synthesis. The increase in HbF which results from a genetically determined increase in F cells appears to ameliorate sicke-cell anaemia or beta thalassaemia. Augmentation of the F-cell population might offer a therapeutic approach to these disorders.