Nuclear enzymatic activities incorporating amino acids into acid-insoluble material were investigated with respect to their differentiation from protein biosynthesis, reaction optima, requisites and localization. The product of the reaction was analyzed with respect to its localization and nature. The nuclear activities are not inhibited by a number of inhibitors for protein biosynthesis. The reaction optima found are similar to those of other residual nuclear syntheses including the stringent dependence on ATP. All naturally occurring amino acids are utilized with different efficiencies. Their incorporation is neither cooperative nor competitive which points to individual incorporation mechanisms. Aminoacylation of tRNA may be involved because the incorporation is RNase-sensitive and aminoacylation of tRNA can be shown under the reaction conditions. The enzymatic activities are exclusively nuclear. Significant activity with unchanged characteristics is released by sonication. 70% of the radiolabel incorporated is exported across the nuclear envelope during the incubation. The residual 30% of the radiolabel is distributed without enrichment in any nuclear subfraction. The products are exclusively of polypeptide nature. Since distinct nuclear proteins (e.g. histones) which are definitely preformed in the cytoplasm by protein biosynthesis become radiolabelled by the incorporation of radiolabelled amino acids, it is evident that the incorporation takes place at preformed polypeptides. This is unequivocally proven by the incorporation of radiolabelled amino acids into exogenous proteins by means of the solubilized nuclear activities. The results indicate that the nuclear activity under investigation reflects a nuclear modification system for polypeptides which may be of similar importance as other post-translational modification systems.