The clinical response to treatment with demeclocycline hydrochloride, erythromycin stearate and ethylsuccinate, tetracycline hydrochloride, methacycline hydrochloride, and troleandomycin was studied in 317 military trainees withMycoplasma pneumoniaepneumonia. Thirty-nine trainees who received penicillin G potassium alone or no antibiotic served as controls. Each of the six antibiotics studied reduced the length of illness as measured by duration of fever, abnormalities in x-ray films of the chest, and hospital stay. Positive pharyngeal cultures forMycoplasma pneumoniaewere obtained following treatment with each of the agents, but no prolongation of illness was noted in association with this post-treatment shedding. For many years there have been questions as to the efficacy of antibiotics in the treatment of primary atypical pneumonia. The early observations, which did not differentiate various etiologic agents, yielded conflicting results.1Meiklejohn et al, using clinical criteria for diagnosis, were able to show that tetracycline is effective in this disease.