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