The effect of the epidermal growth Factor (EGF) on the rate of synthesis of DNA, RNA and proteins in two different kinds of isolated tumor cells have been studied by timing the uptakes of the labeled precursors 3H-thymidine, 3H-uridine and 14C-amino acids. The results obtained from kinetic data have shown that the EGF stimulates the rate of synthesis of DNA, RNA and proteins. However, the activation of the RNA synthesis appears to be an earlier and more specific effect with respect to stimulation of protein and DNA syntheses.