Using ³H-proline as a radioactive tracer, the relationship between the ultrastructural differentiation and the site of protein synthesis has been investigated in developing red blood corpuscles. The general ultra-structure of erythropoietic cells in differentiation after 60 minutes of in vitro labeling has confirmed the results from previous investigations by Bessis, M., Thiery, J. and others. In dividing nuclei more than two-thirds of the labeling were present at the interface between heterochromatin and euchromatin. In less differentiated cells most of the grains in interphase cells was localized over the nucleus. As the cells continued to develope beyond a stage where cytoplasmic density was clearly increased over other cell lines in bone marrow, the majority of grains localized over the cytoplasmic area was decreased in more mature cells, as judged by the density of cytoplasm, and the structural changes in mitochondria, Golgi complex and polysomal configurations. These results show; 1) that the cytoplasm of erythroblast series does not change under in vitro conditions employed in the study; 2) that protein synthesis in the nucleus occurs largely at the interface between euchromatin and heterochromatin in active nuclei; and 3) that cytoplasmic synthesis of proteins continues to take place well into the normoblast stage solong as the physically visible polysomes are present in maturing red blood corpuscles.