The endotoxin was obtained from the autolysate of Pseudomonas aeruginosa. It has very weakpyocine activity against several kinds of strains. The constituents of the endotoxin, a lipopolysaccharide-protein complex and DNA-RNA-polyribose phosphate complex, have no pyocineactivity. Isolated protein moiety (Orig. endot.-P) from the lipopolysaccharide-protein complex, however, has strong pyocine activity against various kinds of strains. The simple protein (LSAS) isolated from the cell wall of the bacteria has the same potency of pyocine activity and has the same spectrum against sensible strains as the protein moiety (Orig. endot.-P) derived from the autolysate. The lipopolysaccharide moiety of the endotoxin has nopyocine activity and it is easily combined with the protein (LSAS or Orig. endot.-P) and masked the pyocine activity of it.The LSAS anti-serum as well as the Orig. endot.-P anti-serum can neutralize the pyocine activities of the two proteins (LSAS and Orig. endot.-P) by each other. Absorption tests reveal the pyocine activities of both proteins are identical. The anti-serum of the lipopolysaccharide moiety can not neutralize any pyocine activities of both proteins. According to the results, it can be concluded that the pyocine isthe protein part of the endotoxin and that the activity may be masked in the cell wall of the bacteria as a result of combining with lipopolysaccharide.