Reagent for detecting early stages of apoptosis. During apoptosis, phosphatidylserine (PS) is translocated from the cytoplasmic face of the plasma membrane to the cell surface. Annexin V has a strong, Ca²ᶧ-dependent affinity for PS and therefore is used as a probe for detecting apoptosis. The Annexin V-FITC conjugate can be used for detection of apoptosis by fluorescence microscopy or by flow cytometry.