PREVIOUS studies have established a relation between the adrenal cortex and the reticulo-endothelial system. Thus, adrenal insufficiency results in an impaired phagocytic capacity of the macrophagic elements of the spleen toward colloidal thorium dioxide (1, 2, 3). In addition, this diminished activity is prevented by the administration of certain adrenal cortical extracts or cortisone (1, 4, 5). In extending these studies, it was deemed advisable to employ radioactive colloidal gold since it can be used in amounts sufficiently low to preclude any toxic effects; and because of its radio-chemical characteristics, its specific accumulation in the phagocytic elements of the reticulo-endothelial organs can be quantitated readily in these small doses (6, 7, 8, 9, 10, 11, 12, 13, 14, 15). Since the entrance of such an agent into the macro- phage occurs normally only through actual ingestion, its use also averts the factor of diffusion, a criticism against the employment of vital dyes for such experiments (16).