Solutions of 10% mannitol and 2% sodium taurocholate were administered i.v. to increase urinary output and bile flow, respectively. A single preparation of purified CEA (6), labeled with â€ ̃ 25 or â€ ̃ @ â€ ̃ I by the chloramine-T method (4), was used throughout the experiments. Dogs were given injections of 10 to 300 X 106 cpm of the radiolabeled CEA, at a specific activity of 10 to 25 pCi/jig, diluted in 20 ml autologous plasma. Timed serial samples of blood, urine, and bile were collected, and the radioactivity was measured in a Nuclear-Chicago y radiation spectrometer. Several samples of urine and bile, collected at specific intervals during the experiments, were chromato graphed on calibrated Sephadex G-200 columns and assayed for their capacity to bind to goat anti-CEA antibody by the ammonium sulfate coprecipitation technique (1). In a number of experiments, the liver, kidneys, lungs, spleen, heart, thyroid, and intestines of the dogs were removed, weighed, and analyzed for accumulated radioactivity. Similar investigations were performed in rabbits in which the plasma disappearance of similar quantities of radiolabeled CEA was studied for 1 hr. In addition, a number of studies were carried out in which the metabolism of the same CEA preparation was examined sequentially in rabbits and dogs. The experimental protocols used are shown in Chart 1. The CEA used in these experiments was shown to be free of aggregates and homogeneous by chromatography on a calibrated Sephadex G-200 column (Chart 2). None of the animals used in these experiments demonstrated anti-CEA activity in their serum.