Intensely blue dye-azoproteins have been prepared by diazotization and coupling of the highly indiffusible blue dye T-1824, Evans blue, with various serum proteins and egg albumin. The products, whether purified by precipitation with alcohol or by chromatography, have a constant dye-to-protein ratio and tests have shown them to be essentially free from unlinked dye. An extremely diffusible dye, echt-säure-blau, has also been coupled to bovine gamma-globulin. These materials are adapted to physiological experimentation. They seem to behave in the bodies of mice like other proteins; they fail to appear in either the bile or urine of normal animals, and they are strongly antigenic. When these soluble antigenic azoproteins are injected into the blood stream of mice for the first time they enter reticulo-endothelial cells in almost every organ of the body; the final distribution is like that of intravenously injected, finely divided particulate matter. The azoproteins appear in the cells which classical immunological studies have shown to be active in removing particulate antigenic materials or bacteria from the blood or body fluids. The Kupffer cells of the liver and sinus and reticular cells in lymph nodes, especially the great mesenteric node, are particularly active in the removal of the blue antigens from the blood, but many other R-E cells are active to a lesser degree. The storage of the antigenic material is in the cytoplasm only; it has not been seen within nuclei, nor has it been seen within cells of the brain. Serological methods disclose that the blue material seen within Kupffer cells of the liver after as long a period as 2 days is still antigenic in its reactions. The blue azoproteins, therefore, serve excellently as tracer antigens, especially since they can be seen directly in fresh and fixed tissue preparations and in the body fluids.