Apoptosis is a highly conserved form of cell death present in all eukaryotic cell types and controlled by multiple genes. Several methods have been developed to quantify apoptosis, but none is adapted for all cell types. It is particularly difficult to reliably assay apoptosis of adherent cells. We describe a new, rapid and reliable flow cytometric method which can be used for quantifiying apoptosis in a sub-population of transiently transfected adherent cells. This technique is based on the detection of transfected cells and the apoptotic sub-population by immunofluorescence and Annexin-V labelling, respectively.