The schedule for synthesis of the various hemoglobins of adult humans during the development of erythroid cells is presently unknown. There is some evidence that the gamma chain of HbA2 is made mostly in nucleated precursors and that the presence of HbF is restricted to a small fraction of erythrocytes, called F-Cells. We propose to inject tagged glycine into rats and humans, and then follow its appearance with time in the globin chains. Because free labeled glycine disappears rapidly from peripheral blood, erythroid precursors are exposed to "pulse-labeling." Appearance of isotope in globin chains in erythrocytes should reflect in reverse order their sequence of synthesis as cells mature. The first chains to be labeled should be those made in peripheral reticulocytes; the next, in marrow reticulocytes; .....; the last, in the earliest precursors. By following the sequence of appearance, one can learn whether globin chain formation and ultimately degradation is coordinate or asynchronous. In the case of the multitude of adult rat hemoglobins, studies with 14C-glycine suggest that several hemoglobins represent postsynthetic modification. For human studies 2-13C, 15N-glycine will be the isotopic tag and it will be detected by gas chromatography/mass spectrometry. It should be of interest to compare data on alpha, beta, gamma and sigma chains from normal human subjects with those from certain variants such as persons with sickle cell disease or thalassemia and high versus low levels of HbF.