Histone biosynthesis was investigated in proerythroblasts and megaloblasts obtained from patients with untreated pernicious anemia. Radioautography, using tritium-labeled L-arginine, L-lysine, and L-trytophane, acrylamide gel electrophoresis of histone fractions, acrylamide gel radioautography, uptake of radioisotopes into lysine-rich and arginine-rich histone fractions, and amino acid analysis of histone fractions, was performed. In untreated pernicious anemia proerythroblasts and megaloblasts, there was a small uptake of arginine and lysine as seen radioautographically, and incorporation into isolated histone fractions was scant. Amino acid analyses showed these histones to be lysine rich. After these erythroid precursors were exposed to cyanocobalamin in vivo or to coenzyme B12 in vitro, there was a marked (up to 100-fold) incorporation of only tritium-labeled L-arginine into erythroid nuclei, as seen radioautographically, and into isolated histone fractions as well. Amino acid analyses demonstrated that the histones had become more arginine rich. These studies indicate that vitamin B12 facilitates the biosynthesis of arginine-rich histones in vitamin B12-deficient erythroid precursors.