This CXCR4 (phospho Ser339) Cell Based ELISA Kit allows for the detection of CXCR4 (phospho Ser339) and the effects that certain stimulation conditions have on CXCR4 (phospho Ser339) expression in different cell lines. Qualitative determination of CXCR4 (phospho Ser339) concentration is achieved by an indirect ELISA format. In essence, CXCR4 (phospho Ser339) is captured by Anti-CXCR4 (phospho Ser339) Antibodies while Dye 1-conjugated and Dye 2-conjugated secondary antibodies bind the Fc region of the primary antibody. Through this binding, the dye conjugated to the secondary antibody can emit light at a certain wavelength given proper excitation, hence allowing for a fluorometric detection method. A monoclonal antibody specific for human GAPDH is included to serve as an internal positive control in normalizing the target RFU values. Anti-CXCR4 Antibody is also included for normalization purposes. The RFU values obtained for the non-phosphorylated target can be used to normalize the RFU value for the phosphorylated target.