The aminoacyl-tRNA and protein synthesizing activities of cell-free systems derived from the livers of mice fed ad libitum and mice fasted overnight were measured. The ‘pH 5 fractions’ from both groups of mice have a similar ability to acylate tRNA. Cell-free systems derived from fasted mice had a reduced protein synthetic activity, but in heterogenous cell-free systems the activity of polysomes from these animals was significantly increased by cytosol fractions from fed mice. These results indicate that fasting affects a factor or factors in the cytosol which are required for incorporation of aminoacyl-tRNA into prot