The biological activity of Odontomyces viscosus, which has been reported to cause periodontal disease in hamsters, was examined. The microorganism was cultured anaerobically in Brain Heart Infusion broth, and the cells were harvested. The washed cells were injected intradermally into the abdomen of rabbits. After 72 hr, a well-defined, firm, raised nodule (about 1.0 by 1.5 cm) with an erythematous border was seen at the injection site. Suspensions of cell wall and cytoplasmic material were injected intradermally, and the lesions appeared only at the site of cell wall injection. The cell walls, which were then treated with trypsin, pepsin, and ribonuclease, again produced the characteristic lesion. These nodular dermal lesions persisted for a minimal time of 10 days. The enzymatically treated cell walls were then hydrolyzed with 1 n HCl, and such hydrolysis up to 1 hr failed to alter the toxic activity of the cell walls. Similar dermal nodular lesions were obtained by injection of enzymatically treated cell walls of strains of Staphylococcus aureus, Streptococcus groups B, C, E, F, K, Lactobacillus casei, and Actinomyces israelii. Treatment with hot and cold trichloroacetic acid solutions and proteolytic enzymes, or with formamide, yielded insoluble fractions which produced the characteristic nodular lesions. The size of the lesion resulting from injection of these fractions was proportional to the amount of the injected material. The active fraction, which does not appear susceptible to hydrolysis by lysozyme, is thought to be cell wall mucopeptide. Histological studies showed skin abscesses due to the toxic reaction; however, in addition to the acute inflammatory reaction, there was local eosinophilia.