The cytosol fractions from resting rat liver and from the liver 1 hour after partial hepatectomy which were not adsorbed on DEAE-cellulose were subjected to gel filtration with a view of elucidating the mechanism of its activating effect on the RNA-synthesizing ability of isolated nuclei of resting liver. The activating effect of this fraction is a complex one and includes both specific and non-specific factors. A subfraction typical for regenerating liver was detected, which activates RNA elongation in a synthesizing system with isolated nuclei of resting liver.