Neutron reflectivity has been used to investigate the interaction between the cationic surfactant dodecyltrimethylammonium bromide (C(12)TAB) and preadsorbed lysozyme layers at the hydrophilic silica-water interface. Reflectivity measurements were carried out with two different concentrations of lysozyme and a range of C12TAB concentrations at a solution pH of 7. A preadsorbed lysozyme layer was prepared by adsorption from 0.03 or 1 g dm(-3) protein solutions, The effect on the adsorbed protein layer structure upon addition of a range of surfactant concentrations, from 0.2 to 14 mM, was determined and the surface excesses of both protein and surfactant within the adsorbed layers calculated. The surface composition of the mixed layers and their structural distributions were identified with the help of the variation of hydrogen/deuterium labeling to the surfactant. It was found that upon increasing surfactant concentration the protein was gradually replaced by the surfactant at the interface. However, a simple replacement mechanism was not observed. Protein removal over the low surfactant concentration range was accompanied by little surfactant adsorption, showing that the desorption was likely to be caused by the formation of highly soluble protein/surfactant complex. At the high surfactant concentration, protein removal was driven by the interplay of the interactions involving the protein, surfactant, and substrate, as evidenced from the coadsorption of surfactant and protein at the interface and the variation of interfacial composition with surfactant concentration. These results, together with previous measurements using sodium dodecyl sulfate (SDS) suggest that for surfactants with the same alkyl chain length the fraction of protein removal is dictated by the nature of the surfactant headgroups. These studies have shown that neutron reflectivity has distinct advantages over other techniques in investigating multicomponent interfacial systems involving biomolecules.