Minor fractions of “residual proteins” from erythroblast-enriched avian erythroid cells were found to incorporate significantly larger amounts of radiophosphate than similar fractions from mature erythrocytes. This higher level of incorporation could not be detected in the bulk, unfractionated residual proteins from cell populations, respectively, enriched in erythroblasts, reticulocytes, or mature erythrocytes, probably because of variable amounts of phosphate-free protein. It was confirmed that when avian erythroid cells incorporated radiophosphate, specific activities of chromosomal “residual proteins” were orders of magnitude greater than those of histones, although levels of alkali-labile phosphate were only a fewfold higher. The phosphate incorporated into the residual proteins was not in the form of free nucleotide, inorganic phosphate, phospholipid or nucleic acid. The prospect of contamination by unextracted histones was reduced by washing the nuclear preparations with acidified organic solvents, in which some histones are soluble. Furthermore, histones and hemoglobin can be distinguished from residual proteins by characteristic mobilities in polyacrylamide gel electrophoresis with sodium dodecylsulfate. The radiophosphate bound by residual proteins was distributed among virtually all of the components resolved by zonal electrophoresis, including bands characteristic of erythrocytes. At least one major radioactive peak, extracted in phenol at pH 9.4, did not migrate with a pronounced band of protein and other components of high specific activities may be resolved from concomitant proteins on sequential electrophoresis in different systems. The role of increased protein phosphorylation in immature erythroid cells is unknown.