In 1970, it was found that sera removed from rats 24 hr after unilateral nephrectomy stimulate the incorporation of 3H-Thymidine into the DNA of incubating rat renal cortex. The same sera did not influence isotope incorporation into the DNA of incubating liver, spleen, and lung tissue. Later, a substance was described in renal tissue that enhanced the action of the renotropic factor in sera. It was proposed that this tissue factor activated the circulating renotropic substance and that lack of this activator in other tissues was responsible for the specificity of the circulating renotropic substance to renal tissue. The present study suggests that this is the case as the addition of the renal tissue factor and sera from uninephrectomized rats can stimulate 3H-Thymidine incorporation into the DNA of rat liver slices.