The participation of a circulating growth factor in the mediation of compensatory renal growth has been hypothesized but not proven. Theoretical considerations predict that in vitro methods could be profitably used to resolve this question, since such methods would allow dissociation of the growth effects of the postulated factor from the complex physiological changes which accompany compensatory renal growth in vivo. 2. A culture system using epithelial cells obtained from the rat kidney is described which is suitable for testing serum from previously unilaterally nephrectomized or sham-operated rats for the presence of a factor mediating compensatory renal growth. The morphology of the cultured cells is compatible with that of proximal tubule epithelium, this being the cell type stimulated to divide in vivo after unilateral nephrectomy. 3. Treatment with serum from rats unilaterally nephrectomized 48 h previously results in consistent increase in uptake of tritiated thymidine by cultures, when compared with control serum from sham-operated rats. Serum from rats unilaterally nephrectomized 18--36 h previously is not consistently stimulatory. 4. There are indications that the differential effect of sera from unilaterally nephrectomized and control animals is due to the presence of a stimulatory factor in the former rather than an inhibitor in the latter. 5. Use of this culture system has confirmed the existence of a serum factor involved in compensatory renal growth.
