Using 64 cultures of virulent Corynebacterium diphtheriae obtained from as many patients, it was detd. that but 41 (64%) were sensitive in vitro to penicillin solns. of 0.1 units per ml. Experience has indicated that if any organisms are resistant to higher cones., there is little or no patient response. The sensitivity of these 64 cultures to freshly prepared penicillin solns. was the same before and after pure O2 had been vigorously bubbled through the solns. for 15 min. Nebulization of 2 1/2 ml. penicillin soln. (2000 U./ml.), over a period of 15 min. 4 times daily, intranasally or intraorally, failed to shorten the period required to rid the nose and throat of virulent C. diphtheriae in 19 carriers below that interval required by 19 controls. For the purpose of the test, a carrier was defined as one who, 10 days after the onset of his diphtheria symptoms, continued to have positive smears or cultures. Concs. of penicillin Na up to 50,000 U./ml. nebulized into the nose were found non-irritating.