The synthesis of the nuclear acidic proteins in immature and mature avian erythroid cells was compared. Synthesis of these proteins was found to occur in both types of cells. However, there are quantitative and qualitative differences in incorporation of labelled amino acids into the electrophoretic patterns of these proteins. There was an overall decline in the specific activity of the proteins as erythrocyte maturation proceeds. In particular, one protein band of molecular weight 81,000 is predominantly synthesizied in the immature cells but not in the mature cells.