A protoplast (P) is situated close to a spheroplast (S). No lable is attached to the remnants of the outer membrane (O), whereas lable (G) is seen attached to the exposed surface of the cytoplasmic membrane (C) of the protoplast, but not to the covered cytoplasmic membrane (C) of the spheroplast (S). These data indicate that the membrane-integrated enzyme exposes immunogenic epitopes towards the periplasm of the cell. (From Eismann, K., Mlejnek, K., Zipprich, D., Hoppert, M., Gerberding, H., Mayer, F., 1995, J. Bacteriol. 177, 6309-6312).