Although the developmental switch from fetal to adult haemoglobin production has been well characterized in terms of protein synthesis, very little is known about its control at the cellular level. In order to determine whether the switch is controlled by environmental factors or programmed into the haemopoietic cell population, we have studied the effects of transplantation of fetal and adult haemopoietic cells into lethally irradiated lambs on haemoglobin synthesis by the transplanted cells. In two lambs which were irradiated but not transplanted, two lambs which were grafted with autologous marrow, and one lamb which was grafted from an allogenic twin, haemoglobin synthesis showed an adult pattern following transplantation, with gamma-chain synthesis never exceeding 7% of non-alpha chain production. Of 11 lambs transplanted with fetal haemopoietic cells, only two showed evidence of engraftment. During the 24-26 d that these animals survived, haemoglobin synthesis showed a predominantly fetal pattern, though there was a gradual increase in beta-chain production of donor origin. This increase occurred earlier than would be expected from the gestational age of the transplanted cells, but more slowly than might be expected if environmental factors were entirely responsible for expression of adult or fetal haemoglobin synthesis.