Preliminary results are reported in an ongoing program to develop automated cytologic examinations for the detection of bladder cancer from exfoliated urinary epithelium. A metachromatic fluorescent dye, acridine orange, was used to stain the cells in suspension in such a way that their nuclei (DNA) fluoresced green in blue light, and their cytoplasm (chiefly RNA) fluoresced red. The intensities of green and red fluorescence per cell were measured for up to 5000 cells per sample by a flow-through cytofluorometer, and differences were found between cell types that served to identify bladder epithelial cells, leukocytes, squamous cells, and other particulates. Benign and malignant bladder epithelial cells differed principally in their cytoplasmic RNA staining intensity, and methods are discussed for amplifying this and other differences that may be of diagnostic value in an automated instrumental system.