A fluorimetric assay using ethidium bromide (EB) was employed to quantify cell death in monolayer cell cultures (MA-104 cells) in situ and isolated cell suspensions (isolated colonic cells and Leishmania). Fluorescence of EB stained cells was measured with a photometer coupled to an inverted microscope for cell monolayers or in a spectrofluorometer for cell suspensions. Dead cells stained with trypan blue were fluorescent with EB in all preparations studied, but the latter gave an unequivocal signal. Staining with EB and fluorescein diacetate was mutually exclusive. The relationship between the number of EB fluorescent cells and the intensity of fluorescence measured in the microphotometer was linear for a large range of cell numbers (1-14000) from different types of preparations. Applicability of the method for measuring living and dead cells in two different time scales (minutes and hours) is shown using MA-104 cell monolayers infected with rotavirus and Leishmania suspensions treated with amphotericin B. The method is fast, simple, sensitive and reliable, enabling quantification of living and dead cells in monolayers and suspensions.