Identifying molecules that serve as markers for cell aging is a goal that has been pursued by several groups. Senescence-associated β galactosidase (SA-βgal) staining is broadly used and very easily detected. β-gal is a lysosomal enzyme strongly correlated to the progression of cell senescence. Here, we describe a simple, fast, and quantitative protocol to quantify SA-βgal activity in cell lysate extracts by a chemiluminescent method using galacton as substrate.