Compensatory renal hypertrophy after unilateral nephrectomy is well known. Among several explanations for such an occurrence, a factor in the Serum of unilaterally nephrectomized rats that causes renal hypertrophy has been described (1-4). However, this hypothesis is not completely accepted, and one report denied the existence of such a factor ( 5 ) . However, a renal hypertrophy effect has been noted after injection into normal rats of serum from bilaterally nephrectomized animals (6, 7).The present study was undertaken to add evidence for or against the presence of a renal hypertrophy factor in the serum of uniand bilaterally nephrectomized rats.Weight and the uptake of tritiated thymidine of the kidneys of assay (or recipient) rats were utilized in this study (8). Thymidine uptake proved to be the most sensitive and useful indicator. Our data confirmed the presence of such a factor in nephrectomized rats. We were unable to observe the effect in the rat assay of sera obtained from uni-and bilaterally nephrectomized dogs and man, and from a patient with severe renal insufficiency. The renal hypertrophy factor appears to be species specific, although an alternative explanation for our observations is possible.Materials a d Methods. Male albino rats of a Sprague-Dawley strain, ranging in weight from 140-150 g, were divided into three groups:Group I (rat donors). Eighty recipient rats were separated into four sets.I -A (normal saline controls). Twenty rats received injections of normal saline. This work was aided by grants from the US.Public Health Service ( H E 07057 and FR-48) and the Otho S. A. Sprague Foundation.