Without removing the culture medium from the cells, add 1/10 volume of 10X Annexin V solution to the medium on the cells. For example, for cells seeded in a 96-well plate in 100 uL per well, add 10 uL of 10X Annexin V to the medium in the well. Immediately mix by gently pipetting half the volume of the well up and down several times, taking care not to introduce bubbles. Scale all volumes proportionally for larger culture vessels.