Interchromat in granules are normal constituents of mammalian cell nuclei. They have been described as ribonucleoproteins on the basis of their staining properties. However, contradictory results have been reported about their resistance to enzymatic and chemical extraction. They have never been specifically labeled with tritiated uridine in spite of several attempts. In this paper, interchromatin granules were investigated using cytochemical techniques. The reacting groups, responsible for interchromatin granules' high affinity for stains, were identified as phosphate groups using histochemical blockades like acetylation and methylation. The staining method of Locke and Huie was slightly modified in order to be used on sections. This technique enables the revealing of very reactive phosphate groups of proteins and not phosphate groups of RNA. The possible role of phosphorylated proteins of interchromatin granules in the metabolism of the cell nucleus is reevaluated.