Epitope tagging is a powerful and versatile strategy for detecting and purifying proteins expressed by cloned genes. Short sequences encoding the epitope tag are cloned in-frame with target DNA to produce fusion proteins containing the epitope tag peptide. Due to their small size, epitope tags do not affect the tagged protein’s biochemical properties. Anti-epitope tag antibodies can serve as universal purification or detection reagents for any tag-containing protein. The T7-Tag antibody is directed against the 11 amino acid gene 10 leader peptide expressed by many translation vectors (MASMTGGQQMG). Because the peptide is the natural amino terminal end of the T7 major capsid protein, the antibody also recognizes T7 bacteriophage. Anti-T7-Tag antibodies are useful reagents to easily identify, detect, or purify T7-Tag fusion proteins from cell lysates.