The patterns of haemoglobin synthesis in erythroid colonies grown from sheep marrow CFU-E of different ontogenic stages have been reported previously (Darbre et al, 1979) but only at a single fixed time point in culture. Culture of adult and late fetal progenitors enhanced gamma chain synthesis whereas culture of earlier progenitors during the 'fetal switch' seemed to increase beta-chain synthesis. In order to try to explain these findings, patterns of growth and of haemoglobin synthesis in the colonies were examined over a time-course of 6 d for all developmental stages. Colonies assessed later in vitro made proportionately less gamma chain at every age. It was concluded that because fetal colonies mature more quickly in vitro than later and adult counterparts, and also because gamma and beta chain synthesis appeared to be asynchronous during sheep colony maturation, the results of assay at a single time point are not strictly comparable. The relevance of these findings to the study of the switch from fetal to adult haemoglobin synthesis is discussed.