Nonenzymatic acetylation of calf-thymus lysine- and arginine-rich histones was demonstrated to occur when these proteins were incubated with [14C]acetyl phosphate and [14C]acetyl adenylate. The levels of acetylation depend on both pH and on reagent concentration. When acetyl [33P]phosphate and acetyl [3H]adenylate were used as reagents, we found neither histone phosphorylation nor adenylylation. Most of the radioactivity of 14C-labeled acetylated histones was recovered as Ne-acetyllysine. Furthermore, only a small amount of O-bound radioactivity was released by the 14C-labeled acetylated arginine-rich histone during treatment with hydroxylamine. Experiments on the acetylation of histones, in the presence of increasing salt concentration, gave different results for the two acetylating agents.