A plasma factor has been hypothesized to initiate compensatory growth of the remaining kidney following unilateral nephrectomy. Evidence supporting the existence of this factor has been obtained in studies of hamster renal cortical cells grown in monolayer tissue culture. These cultured cells resemble renal epithelium in electron micrographs. The incorporation of 3H-uridine into RNA was significantly increased in those cultures incubated in plasma from uninephrectomized hamsters (UP) above those containing sham plasma (SP) at 10 and 15 % plasma in the medium. The uptake of uridine into the cy toplasmic fraction of those cultures incubated for 4, 6, 8, 10 and 24 h and in the nuclear fraction for 2, 4, 6, 8 and 24 h in UP was significantly greater than those incubated in SP. Plasma from partially hepatectomized hamsters did not increase the growth rate of this tissue culture over those cultures incubated in SP. Dialysis of UP does not affect its growth characteristics. Therefore, a renotrophic factor is present in the plasma of uninephrectomized hamsters which increases the uptake of uridine into renal tissue cultures.