The adsorption of lysozyme and sodium dodecyl sulfate (SDS) at the air-water interface has been studied using specular neutron reflection and surface tension measurements. All the results reported in this work were measured at a fixed lysozyme concentration of 0.01 g dm(-3) under varying SDS concentration. The neutron measurements were made on the surface of null reflecting water so that all the specular signals arose from the adsorbed layers. The surface composition of the layer was determined by varying H/D labeling of SDS. The results show a significant enhancement of the surface excesses for both lysozyme and SDS upon addition of SDS (region A), suggesting that the lysozyme/SDS complexes are much more surface active than the unbound species. This observation is consistent with the substantial reduction in surface tension over this region. Although the total surface excesses increase with SDS concentration up to [SDS] = 2.5 x 10(-4) M, the molar ratio of SDS to lysozyme over this region remains constant at 7 +/- 1, showing that the complex adsorbed on the surface has a rather well-defined stoichiometric composition. Increase in SDS concentration above 2.5 x 10(-4) M leads to a fast decline in the total surface excess (region B), but the molar ratio of SDS to lysozyme starts to rise sharply, indicating that the association of SDS has increased the solubility of SDS/lysozyme complex. Further addition of SDS beyond 1 x 10(-3) M results in an almost complete dissolution of the SDS/lysozyme complex into bulk solution, marked by the rise of surface tension (region C). The subsequent drop of the surface tension signifies the adsorption of free SDS molecules. The critical micellar concentration (cmc) for the surfactant in the mixed system was found to be lower than that for pure SDS, showing that SDS micellization was facilitated by polypeptide fragments. Hence above the cmc (region D) the solution contains a mixture of polypeptide-rich and polypeptide-lean aggregates.