The switch from HbA (a&; 4) to HbC (cu&) synthesis was induced by injection of erythrdpoietin into a iamb homozygous for HbA. Serial samples of bone marrow were analyzed to detect the initial commitment of erythroid stem cells (CFU-E) to form colonies which made HbC in vitro, and to detect the initial accumulation of pc-globin mRNA and the onset of HbC synthesis in erythroblasts in vivo. CFU-E-derived erythroid colonies were formed in plasma clot culture at a low erythropoietin concentration, and the relative amounts of PA-and@-globin synthesized were determined after a 24 hr pulse of 3H-ieucine, added after 84 hr in culture. RNA was extracted from nuclei and cytoplasm of “early” and “late” populations of bone marrow erythrobiasts which had been fractionated by Ficoll-Hypaque density centrifugation. The concentration of/3*-and@-globin mRNA was determined by annealing to purified synthetic DNAs (cDNAs) complementary to p* and p” mRNA. No p-giobin was synthesized in erythroblasts or in CFU-E-derived erythroid colonies prior to the injection of erythropoietin. An increase in the concentration of CFU-E in the bone marrow and the appearance of@-globin synthesis in CFU-E-derived colonies were detected 12 hr after the erythropoietin injection. in contrast, pC mRNA was not detected in either “early” or “late” erythroid ceils until 38 hr later. The first measurable PC-giobin mRNA was accompanied by the ap pearance of p-globin synthesis in bone marrow erythrobiasts. Our results suggest that the accumulation of pc-globin mRNA is a relatively late event following induction of HbA to HbC switching by erythropoietin. The expansion of the compartment of erythroid stem ceils and the commitment of CFU-E to p-globin synthesis appear to precede the detectable accumulation of pC mRNA by 24-38 hr.