The introduction of penicillin for treatment of bacterial infections is one of the most important developments in chemotherapy. The relative lack of toxicity of penicillin for most tissues is one of its great advantages. This lack of toxicity was apparent to Fleming1and to Florey and his associates.2It was further evident from studies carried out in our laboratories, which began early in 1941,3that penicillin was a highly antibacterial substance and at the same time possessed very low toxicity for tissue as measured by means of tissue culture methods. Although penicillin is exceedingly effective in treatment of some infections, it is ineffective against many others. One of the essential requirements for successful treatment of bacterial infections with penicillin is to limit its use to infections due to those pathogens which are known to be susceptible. At present the susceptible and the insusceptible organisms are essentially those