SDS-PAGE analysis of the outer membrane proteins (OMP) of Shigella dysenteriae, S. flexneri, S. boydii and S. sonnei showed similar profiles after isolation from Shigella species grown at 30, 37 and 42 degrees C. In S. dysenteriae there was a 25 and a 17 kD polypeptide at 37 degrees C only, compared with the other two temperatures. In all four Shigella spp the major protein band was around 34-38 kD. Antibodies raised in mice against formalinized bacteria gave a strong OMP specific response in both IgM and IgG classes, with IgG1 being the dominant subclass in all the species. Western blotting showed the major OMP to be the predominant antigen which was present in all the three temperatures tested. Cross-reactivity studies of the antigenic OMP of Shigella revealed that the major protein around 38 kD was related in the four species. Biochemical analysis of the partially purified major OMP indicated it to be porin.