The p16-INK4a (Phospho-Ser152) Colorimetric Cell-Based ELISA Kit is a convenient, lysate-free, high throughput and sensitive assay kit that can detect p16-INK4a (Phospho-Ser152) protein expression profile in cells. The kit can be used for measuring the relative amounts of p16-INK4a (Phospho-Ser152) in cultured cells as well as screening for the effects that variuos treatments, inhibitors (ie siRNA or chemicals), or activators have on p16-INK4a (Phospho-Ser152). This kit contains Crystal Violet Staining normalization protocol. It provides a means to normalize Absorbance 450 nm of primary antibodies to cell density of the cells in the same well. This provides an accurate account for stimulations and cell densities during the assay. By comparing the proportional values obtained from the experimental wells to those of the control wells (e.g. untreated cells), it is possible to determine the extent to which the stimulant(s) have affected the expression levels.