The interaction between lysozyme and sodium dodecyl sulfate (SDS) was investigated by dynamic and static light scattering. The lysozyme-SDS system was studied at two different pH values (3.3 and 9.0), at which the net positive charges of lysozyme are 16 and 6, respectively. Furthermore, a comparison was made between the native lysozyme-SDS system and the reduced lysozyme (i.e., lacking its four disulfide bridges)-SDS system at pH 3.3. The concentration of SDS was varied from 0 to 0.55 M. The lowest molar SDS/lysozyme ratio was approximate to 55 because lower ratios result in a precipitate, making the samples unsuitable for measurement. Under the conditions used, (free) SDS micelles and lysozyme-SDS complexes coexist. Based on characteristic differences in the relaxation time distributions, the results obtained at low SDS concentration (<0.1 M) are discussed separately from those obtained at high SDS concentration (>0.25 M). At low SDS concentration, the hydrodynamic radii for the lysozyme-SDS complex in the three different systems are 3.23 +/- 0.04 nm (native lysozyme-SDS, pH 3.3), 3.0 +/- 0.1 nm (reduced lysozyme-SDS, pH 3.3), and 2.61 +/- 0.06 nm (native lysozyme-SDS, pH 9.0). The hydrodymamic radius of (native) lysozyme at pH 3.3 is 1.82 +/- 0.01 nm, and that of reduced lysozyme is slightly smaller (i.e., 1.79 +/-0.01 nm). The rather small hydrodynamic radii of the complexes cannot be accounted for by an unfolded lysozyme molecule. Reducing the disulfide bridges does not result in an increase, but a small decrease of the hydrodynamic radius. The small size can possibly be explained by a complex containing a micelle-like aggregate bound to the protein. At high SDS concentrations, two relaxation modes exist, both representing diffusional species and both containing lysozyme. It is assumed that two different complexes coexist: complex 1 is similar to that obtained at low SDS concentrations and has a compact structure, and complex 2 is a larger complex in which lysozyme probably has a more open, expanded structure, presumably caused by the binding of a greater amount of SDS. The hypothesis of a more expanded lysozyme molecule corresponds well with the change in the observed refractive index increment (partial derivative n/partial derivative c(1vz)) values. The number of lysozyme molecules in the complex does not seem to depend on the SDS concentration and is determined, for the three systems studied in this article, to be similar to 1.