The complexation of a globular protein, beta-lactoglobulin [GRAPHIC] (BLG), and an anionic surfactant sodium dodecyl sulfate (SDS) in aqueous media was investigated using capillary zone electrophoresis, electrophoretic, static, and dynamic light scattering, and small-angle X-ray scattering in a considerably high protein concentration range (0.27 mM < C-P < 3 mM). On increasing the molar concentration C-R of the surfactant, cooperative binding of SDS to BLG starts at C-R/C-P approximate to 1; the BLGSDS complex consists mainly of the BLG dimer and approximately 20 SDS molecules, where BLG takes a compact conformation similar to that of the native BLG up to C-R/C-P 20. At CR/CP higher than 30, the BLG dimer in the BLGSDS complex dissociates into a unimer, but the dissociated BLG unimer still takes a compact conformation at least at 30 < C-R/C-P < 65.