The Skp protein of Escherichia coli has been proposed to be a periplasmic molecular chaperone involved in the biogenesis of outer membrane proteins. In this study, evidence is obtained that Skp exists in two different states characterized by their different sensitivity to proteases. The conversion between these states can be modulated in vitro by phospholipids, lipopolysaccharides and bivalent cations. Skp is able to associate with and insert into phospholipid membranes in vitro, indicating that it may associate with phospholipids in the inner and/or outer membrane in vivo. In addition, it interacts specifically with outer membrane proteins that are in their non-native state. We propose that Skp is required in vivo for the efficient targeting of unfolded outer membrane proteins to the membrane.