Isolated calf thymus nuclei werein vitro methylated with S-adenosyl-L-methyl-14C methionine, and the proteins were fractionated according to their solubilities. Histone fraction (H2SO4-soluble fraction) contained approximately 60% total radioactivity incorporated, while “residual protein” which was H2SO4-insoluble contained the remaining radio-activity. The “residual protein” was further fractionated into various acidic proteins, which contained very little of the radioactivity. However, the protein fraction eluted from DEAE-cellulose with 0.5 N NaOH contained the largest amount of radioactivity. This protein was found to be basic in nature by amino analysis.