Surfactants are used as additives in some protein separations and crystallization procedures, but the mechanism of their action is still poorly understood. By measuring the osmotic second virial coefficients of lysozyme solutions by static light scattering, we show that small amounts of anionic surfactants of varying molecular weight always make the protein-protein interactions in solution more attractive and that both the charge of the headgroup and the length of the hydrophobic tail mediate the interactions. The same surfactants also modify lysozyme crystallization and promote formation of twinned phases with gross morphologies different from those seen in the absence of surfactant, some of which display remarkably structured patterns on a micrometer scale. The surfactant effects are, however, often only kinetic, as the phases obtained initially recrystallize slowly into large stable crystals. These crystals are of excellent X-ray diffraction quality and resolution (up to 1.4 Å). Their symmetry, unit cell dimensions, crystal contacts, and protein backbone conformation are the same as those commonly observed for lysozyme, but sometimes occur at atypical pH values. The data suggest new techniques for modification of protein crystallization.