Annexin V binds is a phospholipid phosphatidylserine (PS) binding protein. In the earlier stages of apoptosis, PS is translocated to the outer sections of the cell plasma membrane and is exposed to the external cellular environment. The mark of the late stages of cellular death is the loss of membrane integrity. Cells can be stained with Annexin V conjugated to a fluorochrome together with 7-AAD, which is excluded from viable cells with intact membranes. This can be used to distinguish cells that are viable (Annexin V and 7-AAD negative), early apoptotic (Annexin V positive and 7-AAD negative), and those that are dead or in late apoptosis (Annexin V and 7-AAD positive). Annexin V binding alone cannot differentiate between necrotic and apoptotic cells. The Annexin V binding buffer is formulated for flow cytometric staining of apoptotic cells with conjugated Annexin V. The buffer is provided ready to use.