A method of passive hemagglutination is described that made it possible to detect specific circulating anti-PPD [purified protein derivative] antibodies (titers 1/160 to 1/5, 120) in 203 cases of postprimary active tuberculosis. There were 5 "false-positive" reactions in 80 cases of nonspecific lung disease. No hemagglutination was found in the sera from 22 cured tuberculous patients, and 76 subjects with positive Mantoux reactions but no demonstrable tuberculous disease. The antibody titers fell to insignificant levels during 3 to 5 months'' treatment of infectious lesions. Antipolyo-sidic and antilipidic circulating antibodies were demonstrated.