IT has been pointed out1,2 that, in the dog, bilateral nephrectomy abolishes erythropoiesis, and it was suggested that the kidney produces a factor stimulating red blood cell formation. The experimental result did not definitely exclude retention of some toxic substance that inhibits the erythropoiesis, since the nephrectomized animals were still uraemic (blood urea level at about 300 mgm. per cent) despite peritoneal dialysis. © 1958 Nature Publishing Group.