A sensitive method for quantification of cells undergoing apoptosis that permits the simultaneous measurement of dual-color cell surface immunofluorescence is presented. Unfixed cells are stained with 7-amino-actinomycin D (7-AAD) for discrimination of live from early apoptotic cells and from cells which have lost membrane integrity (late apoptotic or necrotic, dead cells). Owing to its spectral characteristics 7-AAD can be combined with fluorescein-isothiocyanate (FITC) and phycoerythrin (PE) cell surface staining. After staining, the samples can be treated with paraformaldehyde (PF) solution to eliminate the risk for exposure of laboratory personnel to biohazardous agents and to preserve the cells through fixation for later analysis on the flow cytometer. The value of the method is shown on the measurement of apoptosis in human thymocytes and in human peripheral blood mononuclear cells (PBMC) exposed to various inducers of active cell death. The method is validated by fluorescent activated cell sorting in combination with morphologic examination of the sorted cells. The technique we are presenting is particularly valuable in a clinical setting because it allows rapid multiparameter analysis of apoptosis in combination with cell surface phenotype on biohazardous samples with single laser instrumentation.