The digestion of solidified blood-serum by certain sporing bacilli is well known, and Bienstock (1899) and Rettger have shown that B. putrificus and other obligate anaerobic sporing bacteria destroy protein and cause putrefactive changes. Hardly any observations have been made, however, to determine whether aerobic, non-sporing bacilli and cocci have any power to break down proteid, although it is often assumed they can do so. Rettger (1906) found that bacilli of the colon group did not set up putrefactive changes in a mixture of egg and meat protein; he made no quantitative experiments as to whether any destruction of protein took place, and Dr C. J. Martin informed me that he has for long noticed that preparations of pure proteins, when left exposed to air, did not appear to be broken down by bacteria, although moulds were able to grow in them.