This GTPase Activating Protein (phospho Ser387) Cell Based ELISA Kit allows for the detection of GTPase Activating Protein (phospho Ser387) and the effects that certain stimulation conditions have on GTPase Activating Protein (phospho Ser387) expression in different cell lines. Qualitative determination of GTPase Activating Protein (phospho Ser387) concentration is achieved by an indirect ELISA format. In essence, the GTPase Activating Protein (phospho Ser387) is captured by Anti-GTPase Activating Protein (phospho Ser387) Antibodies which in turn are detected by HRP-conjugated secondary antibodies. Through this binding, the HRP enzyme (conjugated to the secondary antibody) can catalyze a colorimetric reaction upon substrate addition. Due to the qualitative nature of this GTPase Activating Protein (phospho Ser387) Cell Based ELISA Kit, multiple normalization methods are described: 1) Anti-GAPDH Antibody is included to serve as an internal positive control in normalizing the target absorbance values. 2) Following the colorimetric measurement of HRP activity via substrate addition, the Crystal Violet whole-cell staining method is used to determine cell density. After staining, the results can be analyzed by normalizing the absorbance values to cell amounts, by which the plating difference can be adjusted. 3) Anti-GTPase Activating Protein Antibody is provided for normalization purposes. The absorbance values obtained for the non-phosphorylated target can be used to normalize the absorbance values for the phosphorylated target.