The acridine orange (AO) fluorescence method for the diagnosis of cancer, developed by L. von Bertalanffy, utilizes increased amounts of cytoplasmic ribonucleic acid in malignant cells to demonstrate them in brilliant reddish orange colors. Many authors have studied the method for the cytodiagnosis of cancer with materials from the female genital truct, gastric washings, and sputurn. Coriphosphine O (CO) staining has been used conventionally for the use of RNA marker on tissue culture cells, frozen sections, and other specimens. However no practical works have been reported for the cytodiagnosis of cancer with CO staining. In this paper we studied the representation of PNA in malignant cells to test the diagnostic reliability of CO staining, comparing with those of stained independently by AO fluorescence method on different smears from the same specimens.