Five strains of Cellulomonas (ATCC 482, ATCC 488, ATCC 491, ATCC 15724 and ATCC 21399) were evaluated for toxicity. Extracts of cells in both water and oil as well as the cell free spent broth injected into fertile chicken eggs were not lethal to the embryos. Viable organisms inoculated in rats, mice and rabbits by oral, subcutaneous and intraperitoneal routes respectively, did not cause generalized or localized infection nor abnormal changes in the vital organs. The sexual maturity, fertility and the vital organs of adult rats as well as the vital data of their progeny were not affected by feeding 10% protein from all strains.