The specific binding of proteins to functionalized lipid monolayers on aqueous subphases was characterized by neutron reflectivity and fluorescence microscopy measurements. Due to the high affinity and high specificity of their noncovalent interaction, streptavidin (SA) and biotin (vitamin H) were chosen as a model system to investigate the structural characteristics of a recognition process on a molecular length scale. Changes in the neutron reflection from the surfaces of NaCl aqueous (H2O or D2O) protein solutions (10(-8) M SA) were used to monitor the interaction of the protein with a monolayer of a biotinylated lipid in situ. Refinement of the reflectivity data and independent fluorescence microscopic observation of the interface using FITC-labeled SA showed that the protein forms macroscopically homogeneous (and presumably crystalline) domains covering a large portion of the surface. Moreover, the neutron reflection experiments clearly showed the formation of a monomolecular protein layer with an effective thickness, dp = 43.7 +/- 2 A. The area per protein molecule occupied in the film was A0 = 2860 +/- 200 A2 and nw = 260 +/- 100 water molecules were associated with each protein molecule. Quantitative binding was found to occur at biotin surface concentrations as low as 1 molecule/1,250 A2 (compared with approximately 1 molecule/40 A2 for dense packing). This study demonstrates the application of a promising new tool for the systematic investigation of molecular recognition processes in protein/lipid model systems.