Phosphate is taken up by erythrocytes in normal goose blood and incorporated into purified, characterized histones. The firmly bound radiophosphorus was not in the form of inorganic phosphate, free nucleotide, or phospholipid. That it was not due to contaminating nucleic acid or nonhistone protein was shown by chromatographic and electrophoretic localization.Most of the histone-bound radioactivity was recovered in slightly lysine-rich histone IIb1 (or f2a2) and in erythrocyte-specific histone V (or f2c), with the highest specific activity in the former. In these proteins the incorporated radiophosphorus was found only in phosphoserine.The differential incorporation did not correspond to alkali-labile phosphate contents, or to synthesis of the different proteins, and may reflect at least two different roles of phosphorylation in cell maturation and control of gene expression.