The effects of sera from uninephrectomized rats or sham-operated rats on the facilitation of 3H-thymidine incorporation into the cells were examined in the primary monolayer culture of rat renal cortical cells. At serum concentrations ranging from 0.25 to 5.0%, a significant and identical stimulation of the thymidine uptake was seen with both sera. The difference in facilitating the thymidine uptake between the sera from the uninephrectomized and sham-operated rats was significant at a serum concentration of 10%. The compensatory renal growth after uninephrectomy may not be due to the simple elevation of the growth factors that are present in the normal serum, but may be due to the renotropin induced by the reduced renal mass.