The efficacy of demethylchlortetracycline in primary atypical pneumonia associated with Eaton agent was studied by a double-blind procedure. By a process of randomization, 290 bacteriologically negative patients with pneumonia were distributed between a test series who received 0.9 gm. of the drug per day for 6 days and a contratest series who received a placebo. The two series were comparable as to duration and severity of the illness when the patients were admitted to the study. The drug significantly reduced the duration of fever, rales, cough, and fatigue in Eaton pneumonia. It stopped the progression and accelerated the clearing of pulmonary infection.