Acridine Orange has been employed extensively as a cytochemical stain and has shown to stain differentially DNA and RNA, and double-restranded nucleic acids in situ . Acridine Orange can either intercalate into double helical nucleic acids (green fluorescence at 530 nm), or bind electrostatically to phosphate groups of single-stranded molecules (red fluorescence at 640 nm). This unique characteristic makes acridine orange useful for cell-cycle studies. Acridine Orange staining of unfixed cells may be used as a simple, fast means of obtaining information on cell ploidy levels and cell cycle status from DNA measurements (green fluorescence), and cell transcriptional activity from RNA staining (red fluorescence), in human and murine cells lines, peripheral blood and bone marrow specimens from patients with leukemia and mitogenically (phytohemagglutinin) or antigenically (mixed lymphocyte culture) stimulated human peripheral blood cultures.