THE association of desoxyribose nucleic acid with the nucleolus has been reported in a number of different types of cells. Hyden1 in nerve cells, Davidson2 in the rat liver cell and Koller3 in tumour cells, find that particles or granules giving a positive Feulgen reaction occur on the periphery of the Feulgen-negative nucleolus. Caspersson4 has identified these granules with what he calls "nucleolus-associated chromatin". He assigns to them the important function of the secretion and deposition of nucleolar substances and a part in protein synthesis in the cytoplasm. Koller and Davidson believe that this nucleolus-associated chromatin forms an integral part of the nucleolar structure and contributes to the formation of the chromocentre and the nucleolar organisers.