When the nuclei isolated from Novikoff hepatomas were incubated in vitro with S-adenosyl-L-[3H-methyl]methionine and the nuclear proteins were subsequently fractionated, histones were found to have incorporated the radioactivity of a four-to-five times higher rate than those of normal rat or tumor-bearing host rat liver nuclei. This observed increase of histone methylation rate in Novikoff hepatoma nuclei was mainly due to the methylation of two histone subfractions H3 and H4. On amino acid analysis, it was found that tumor histones had much higher proportion of epsilon-N-trimethyllysine in comparison with normal or host liver histones, thus shifting the relative ratio of the amounts of epsilon-N-mono, epsilon-N-di- and epsilon-N-trimethyllysine. This was also reflected in the observation of increased epsilon-N-trimethyllysine levels in the serum of the Novikoff hepatoma-bearing animal.