Kit for measuring apoptosis with high sensitivity by Dual Color Flow Cytometry in mammalian cells. Br-dUTP is more readily incorporated into the genome of apoptotic cells than are larger ligands like fluorescein, biotin, etc. This greater incorporation coupled with the use of a fluorescein labelled anti-BrdU monoclonal antibody (Prod. No. ALX-804-197L) gives rise to a stronger signal in flow cytometry.