The acridine orange (AO) binding to the deoxyribonucleoprotein (DNP) complex of individual normal lymphocytes, infectious mononucleosis (IM) lymphoid cells and leukaemic blast cells was determined by microfluorometry. Increased AO binding to DNP was found with increasing cell density on the slide and was similar in the different cell populations. staining Only in exceptional cases of IM, the AO binding was high irrespective of cell density. The increased AO binding was due to changes in the DNP complex, which might be induced by the release of macromolecular substances from the cells. The results show that in clinical cytological studies, the fluorescence intensities cannot be used for the discrimination between normal and malignant cells.