Equimolar mixed monolayers of dioctadecyl glycylglycinamide amphiphile (2C(18)BGly(2)NH(2)) with other functional amphiphiles bearing guanidinium, pyridine, and alcoholic OH groups were prepared on water, and the binding of aqueous dipeptides to these monolayers was investigated by pi-A. isotherm measurement, FT-IR spectroscopy, and XPS elemental analysis. The binding behavior of GlyLeu to the mixed monolayer of 2C(18)BGly(2)NH(2) and guanidinium amphiphile (2C(18)BGua) was analyzed by a Langmuir isotherm to give a saturation guest/amphiphile ratio (alpha) of 0.46 and a binding constant (K) of 6400 M-1. The former value indicates that the binding site for one GlyLeu molecule was formed cooperatively by the two monolayer components. The binding constant is much enhanced relative to those observed for the 2C(18)BGly(2)NH(2) single-component monolayer (35 M-1) and an equimolar mixed monolayer of 2C(18)BGly(2)NH(2) and benzoic acid amphiphile (2C(18)BCOOH) (475 M-1). When the second amphiphile was replaced with a pyridine amphiphile (2C(18)Py) or with an alcohol amphiphile (2C(18)OH), binding constants for GlyLeu were lowered to 124 and 43 M-1, respectively. The enhanced binding in the former is attributed to strong guanidinium-carboxylate interaction upon C-terminal guest insertion and stable antiparallel hydrogen bonding among peptide chains. The binding of a second dipeptide, LeuGly, to the mixed monolayer of 2C(18)BGly(2)NH(2)/2C(18)BGua gave a K value (2170 M-1) that is only one-third of that of GlyLeu. The difference is apparently related to the disposition of the hydrophobic side chain of the Leu residue in the C-terminal insertion. Thus, size matching of side chains of amino acid residues in host and guest determines selectivity of binding. Guest dipeptides are bound to the host most efficiently when the separation of host peptide chains is suited for the formation of strong hydrogen bonds between host and guest.