Several reports have already appeared presenting evidence that aureomycin is an effective therapeutic agent in primary atypical pneumonia.1All have described the course of the disease in relatively small groups of patients treated with aureomycin. None have presented data on comparable numbers of patients ill during the same period of time who did not receive aureomycin. While the remarkable uniformity of the therapeutic response constitutes a strong argument for the effectiveness of the drug, a certain amount of doubt persists because the course of the disease is exceedingly variable. Primary atypical pneumonia is exceptionally well suited for controlled evaluation of therapeutic agents. In certain population groups cases may occur in large numbers. The febrile course is long enough in a proportion of the patients to permit the demonstration of a definite shortening of the disease. The fatality rate in young persons is extremely low, making it justifiable to withhold