Lipopolysaccharides (LPSs) were extracted by phenol-water from three oral strains of Selenomonas. The preparations were tested for the ability to induce a blastogenic response in cultures of spleen cells from normal and nude BALB/c mice, to activate guinea pig complement and the clotting enzyme system of Limulus polyphemus amoebocytes, and to kill Actinomycin-D treated mice. The capacity of the three LPSs was comparable to that of enterobacterial LPS.