A complex between sodium dodecyl sulfate and a protein polypeptide, derived from bovine serum albumin through reduction and carboxyamidomethylation, was studied by small-angle X-ray scattering (SAXS). The radius of gyration increased from 4.1 to 5.1 nm with the increase in the buffer concentration of sodium phosphate buffer, pH 7, from 10 to 200 mM at 25-degrees-C. The pair distance distribution functions, P(r), showed that the complex contained a few spherical micelle-like clusters as the major substructure. This structural feature is consistent with both the necklace model proposed by Shirahama et al. (J. Biochem. 1974, 75, 309) and the protein-decorated micelle structure model proposed by Ibel et al. (Eur. J. Biochem. 1990, 190, 311). Simulation of the P(r) function by using a spherical three-layered model suggested that the radius of the cluster increased from 2.6 to 2.8 nm with the buffer concentration and the difference in electron densities between the inner and outer regions of each micelle-like cluster was enhanced.