A lectin-induced orientation change of a helical glycopeptide in lipid bilayer membranes was studied. Glycopeptides composed of hydrophobic nona-(G8) and pentapeptide (G4) with a fluorescent probe at the N-terminal and a lactose unit at the C-terminal were synthesized. The glycopeptides were incorporated into lipid bilayer membranes with the lactose unit exposed to the aqueous phase and the peptide chain buried in the membrane. G8 takes a partially helical structure in the membrane, while G4 an irregular structure. Upon binding of lectin to G8 held in the membrane of DPPC liposome, enhancement of fluorescence intensity of the N-terminal anthryl group, reduction of fluorescence quenching of the anthryl group with acrylamide, and increase of CF-leakage from the DPPC liposome were observed. G8', which lacks the O-anthryrlmethylserine residue from G8, formed a voltage-dependent ion channel in BLM experiments. The frequency of single current fluctuations induced by G8' incorporation increased with addition of lectin. These results indicate that the peptide segment of G8 prefers taking a more perpendicular orientation to the membrane upon association with lectin.