To better understand the mechanisms that regulate p120-catenin (p120) and E-cadherin function, we are systematically generating phospho-specific monoclonal antibodies (MAb) to the major p120 phosphorylation sites. p120 has emerged recently as a master regulator of E-cadherin stability and an important modulator of RhoGTPase activities. A number of phosphorylation sites have been identified, but none have as yet been linked to specific regulatory roles. Here, we describe a novel phospho-specific monoclonal antibody to the major PKC-induced p120 phosphorylation site, phospho-serine 879 (pS879). With a few exceptions, p120 MAb pS879 is remarkably specific for the phosphorylated S879 epitope and works effectively in common applications such as Western blot analysis, immunoprecipitation, and immunofluorescence. p120 MAb pS879 should facilitate efforts to identify the role of S879 phosphorylation and to map signaling pathways that modify p120 function through activation of PKC.