ARMSTRONG1 has shown that biological fluorochrome acridine orange under controlled conditions of pH differentiates in fixed tissues betwen deoxyribo- and ribo-nucleic acids, the former showing green fluorescence and the latter red. Schummelfeder, Ebschner and Krogh2, who examined the nature of this specificity, have shown that acid or heat treatment of fixed tissue, prior to its staining by acridine orange, changes the green-yellow fluorescence of the nuclear matter to a copper-red colour similar to that given by ribo-nucleic acid in the cytoplasm. From their observations the authors suggested that the green and red shades of fluorescence of acridine orange are associated with high and low molecular-weight forms of the nucleic acids. The deoxyribonucleic acid within the nucleus, however, is characterized by: (a) high degree of polymerization and (b) high level of orientation, which includes its double-stranded form and the structural organization within the chromosomes. The inquiry conducted by Schummelfeder et al. does not indicate the level of depolymerization necessary for the change in fluorescence. It does not also completely eliminate the significance of orientation in the specificity of greenish fluorescence usually shown by chromatin matter within the nucleus stained by acridine orange.