The KT3 tag is an 11-amino acid peptide epitope with the sequence, KPPTPPPEPET, that was originally derived from the large T-antigen of Simian virus 40 (SV40). It can be engineered onto the N- or C- terminus of a protein of interest. Antibodies that target the KT3 sequence may be a useful tool for immunological detection and analysis of KT3-tagged fusion proteins.