The Strep-tag II is an eight-residue minimal peptide sequence (Trp-Ser-His-Pro-Gln-Phe-Glu-Lys) that exhibits intrinsic affinity toward streptavidin and can be fused to recombinant proteins in various fashions. We describe a protocol that enables quick and mild purification of corresponding Strep-tag II fusion proteins--including their complexes with interacting partners--both from bacterial and eukaryotic cell lysates using affinity chromatography on a matrix carrying an engineered streptavidin (Strep-Tactin), which can be accomplished within 1 h. A high-affinity monoclonal antibody (StrepMAB-Immo) permits stable immobilization of Strep-tag II fusion proteins to solid surfaces, for example, for surface plasmon resonance analysis. Selective and sensitive detection on western blots is achieved with Strep-Tactin/enzyme conjugates or another monoclonal antibody (StrepMAB-Classic). Thus, the Strep-tag II, which is short, biologically inert, proteolytically stable and does not interfere with membrane translocation or protein folding, offers a versatile tool both for the rapid isolation of a functional gene product and for its detection or molecular interaction analysis.