A “bacterial surface substance,” which contained cell wall and probably the cytoplasmic membrane, was prepared by disintegrating the intact cells of Pseudomonas aeruginosa with carborundum or by sonic oscillation. This substance was dissociated by various chemical and enzymatic treatments. The immunochemical and biologic properties of the fractions obtained were investigated and compared with the lipopolysaccharide-protein complex (Component I) isolated from the autolysate. The lipopolysaccharide of Component I was found in the nondialyzable, soluble fraction (LS) of lysozymedigested “bacterial surface substance”; however, it was not found in the protoplast. By treatment of the LS with n-propanol and lower phase solvent, a simple protein was isolated, which proved to be homogeneous by zone electrophoresis and ultracentrifugal analysis (s20,w, 1.7 S). By similar treatment a protein was isolated from the lipopolysaccharide-protein complex, which was shown to be homogeneous by zone electrophoretic and ultracentrifugal analyses (s20,w, 1.1 S). Cross-precipitin tests revealed that the two proteins were serologically identical. Biologically they had the same potencies in the Shwartzman phenomenon and pyrogenic reaction. Almost all of the ribonucleic acid present in the protoplast surface substance was liberated by lysozyme and Nagarse. Chemical studies on the substances dissociated by lysozyme and Nagarse treatment of “bacterial surface substance,” revealed the existence of a mucoprotein layer. By immunochemical, biologic and morphologic analyses the surface structure of the bacteria was divided into several components.