In order to further characterize the modulation of non-alpha-chain synthesis during the maturation of erythro- blasts, we studied globin chain synthesis in individual erythroid clones of different degrees of maturity. Cultures of neonatal and adult BFU-Es were inspected at desired intervals and clones classified according to maturity as young or mature were lifted for biosynthesis. Young clones appeared to be at the onset of hemoglobin formation and were composed of mostly early basophilic erythroblasts. Mature clones were at the peak of hemoglobin formation, had intense red color, and were composed of strongly acidophilic erythroblasts. Young clones had higher γ/γ + β chain synthetic ratios compared to the γ/γ + β ratios in the mature clones of the same culture day; furthermore, young clones had higher γ/γ β ratios compared to the γ/γ + β ratios in mature clones of later culture days. These data strongly suggest that there is an asynchrony in gamma and beta chain synthesis during maturation, with gamma chains preferentially being synthesized at the onset of hemoglobinization of the erythroblast. In these experiments, the relative synthesis of the two molecular species of gamma chains, the Gγ and Aγ chains, was also determined in individual clones. When the measurements in neonatal and adult erythroid bursts were analyzed together, a strong positive correlation of γ/γ + β to g γ/Gγ + Aγ ratios was obtained, in support of the concept of synchronous γ→β and GγAγ switching during development. However, at any given level of γ/γ + β ratio, the adult cells express a higher Aγ synthesis than the neonatal cells, suggesting that there is no strict quantitative relationship between γ/γ β and Gγ/ Gγ + Aγ synthesis. The developmental stage of the cells may influence the program of relative synthesis of Gγ and Aγ chains stronger than the ratio of γ to β chain synthesis. In both the neonatal and adult BFU-E cultures, the mean Gγ/Gγ + Aγ synthetic ratio in the immature erythroid bursts was almost identical to the mean Gγ/Aγ + Aγ ratio in their mature counterparts. This suggests that the program of Gγ and Aγ chain synthesis in the cell is not significantly affected by the change in relative expression of gamma and beta chains, which occurs during the maturation of the cell.