Senescence is thought to be a tumor suppressive mechanism and an underlying cause of aging. Senescence represents an arrested state in which the cells remain viable, but not stimulated to divide by serum or passage in culture. Senescent cells display increase of cell size, senescence-associated expression of β-galactosidase (SA-β-Gal) activity, and altered patterns of gene expression. The Senescence detection kit is designed to fluorescently detect SA-β-Gal activity in cultured cells by FACS. The SA-β-Gal is present only in senescent cells and is not found in pre-senescent, quiescent or immortal cells.