Comparison of polyacrylamide gel electrophoretic protein profiles of four isogenic sets of virulent phase I Sh. sonnei strains and their avirulent phase I cells revealed no differences in outer membrane protein composition between virulent and avirulent derivatives. However, significant qualitative and quantitative differences were found in composition of major outer membrane proteins between strains of different origin. All four strains tested contained one major protein of 33K. This protein was susceptible to proteolytic enzymes and was found to be heat modifiable. Other major proteins of 35K and 37K present in three strains and 36K present in one strain were identified as peptidoglycan associated proteins.