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Fusion protein technology has become an important tool for solving numerous problems linked to recombinant protein production. The properties of the additional tag facilitate identification and provide a one-step purification procedure of the fusion protein by passing cell extracts or supernatants through columns of an appropriate matrix. FLAG peptide allows elution under non-denaturing conditions. Several antibodies against FLAG peptide have been developed. One antibody, M1, binds the peptide in the presence of bivalent metal cations, preferably Ca ²⁺ . Elution is effected by chelating agents. Another strategy is competitive elution with excess of free FLAGe peptide. Antibodies M2 and M5 are applied in this procedure. The Flag-tag is first described as a calcium-dependent epitope of a monoclonal antibody. It is a highly acidic octapeptide which can be N-terminally fused to the protein of interest. As a very hydrophilic peptide the Flag–tag has a high surface probability. Flag-fusion proteins can be captured by an immunoaffinity column in the presence of Ca ²⁺ and eluted byEDTA at low concentrations, neutral pH and thus, nearly physiological conditions.