We examined gamma-globin-chain biosynthesis by adult and umbilical cord blood or erythropoietic bursts in methylcellulose clonal culture and gamma-chain synthesis by cord blood reticulocytes. Globin chains were labeled with 14C-amino acids and guantitated by using autoradiography or fluorography. Alpha, beta, and G gamma and A gamma chains were separated by isoelectric focusing in polyacrylamide gels containing 8 M urea and 3% Nonidet P-40 (a nonionic detergent). Time course examinations of the gamma-chains synthesized by the bursts revealed no changes in the G gamma:A gamma ratio between days 10 and 18 of culture. The ratio of G gamma/(G gamma + A gamma) in cultures of adult circulating erythroid precursors was 0.38 +/- 0.09, which corresponds to the known ratio in adult peripheral blood erythrocytes. The relative G gamma-chain biosyntheses in the cord blood bursts and reticulocytes were 0.56 +/- 0.02 and 0.66 +/- .008, respectively. Both are intermediate between the accepted newborn and adult ratios. Natal erythropoietic precursors appear to be in the transitional stage with respect to the switching of G gamma-A gamma ratios.