Phosphorylation of tyrosine residues of various proteins plays an important role in regulation of the functional activities of proteins within the cell. Antibodies to specific tyrosine-phosphorylated peptides are a valuable tool for analysis of the localization and function of individual phosphoproteins. This unit describes the methods used to produce and screen monoclonal and polyclonal antibodies for specificity for only the phosphorylated state of a peptide.