This affinity-purified antibody is suitable for use in ELISA and western blotting assays. The antibody reacts specifically with phosphotyrosine and shows minimal reactivity by ELISA and western blotting with proteins containing phosphothreonine or phosphoserine residues. The antibody reacts with free phosphotyrosine, phosphotyrosine conjugated to carriers such as thyroglobulin or BSA, and detects the presence of phosphotyrosine proteins of both unstimulated and stimulated cell lysates. Although not tested, this antibody is likely functional in RIA, flow cytometry and immunohistochemistry.