Abstract : The purpose of this research is to determine the crystal structure of the staphylococcal enterotoxins (SEs) by x-ray diffraction and protein modeling techniques and to correlate the 3D structures with their biological activities, in order to provide essential data for use in vaccine, or other effective counteractant, development. Several structures were determined in the project period covered by this report. These include the crystal structures of the following by x-ray diffraction: SEC2, form III of SEB, a mutant (F44-> S) of SEB, and a complex of SEB cocrystallized with lactose. Based on the concept that all SEs possess a common main chain folding motif, SEE and SEA were modeled from the empirical SEB structure. Staphylococcal enterotoxins, Crystal structure, Superantigen, Protein crystallography, X-ray diffraction.