Twenty-three [female] white rats were kept on a standard diet for 14 days; 5 were then killed by decapitation; 6 were killed after a further 5 days on water only; and 12 were killed after 5 days on water and further periods (1-5 days) on the diet. Pieces of their livers were fixed in-Carnoy''s fluid or by freezing and drying. Paraffin sections were photographed in u.-v. light at 2570 and 2750 A. In the liver, cytoplasm of animals fed on protein-containing diet, but not of starved animals, strongly absorbing inclusions were shown, morphologically identical with protein inclusions. From comparison with macrochemical data, it is deduced that the inclusions are at least partly protein that contains nucleic acids, probably of ribose type. Therefore the specific part of the gallocyanine-chromalum staining depends in liver cells on proteins containing nucleic acids.