AS HAS been pointed out by Thomas (1952, 1953), cortisone has been L demonstrated to reduce the resistance of mammals to bacteria, viruses, fungi, and bacterial toxins. In discussing the various methods whereby cortisone might act to produce these results, the author suggests the conclusion that the effect must be on the reticuloendothelial system (RES). At about the same time, it was pointed out that cortisone appeared to depress several functions of the RES if given in sufficient dosages (Heller, 1953). This paper concerns itself with further investigation into the specific nature of the effect of cortisone on RES functions involved in infection. MATERIALS AND METHODS Relying on the exclusive ability of the RES to take up negatively charged colloids (Goulian, 1953), radio-active colloidal chromium phosphate labeled with P32 was prepared for use as an indicator according to the technique of Jones (1944). Heterogeneity of particle size (demonstrated by electron micrography)