BCG tuberculin is prepared from BCG cultures grown as a matter of routine on Sauton's medium to obtain Calmette's vaccine. The BCG organisms are allowed to grow for 11 to 12 days prior to the preparation of the tuberculin. In intradermal tests in BCG-infected guinea pigs this BCG tuberculin causes reactions at an earlier stage than equivalent amounts of commercial tuberculin. In those animals in which initially positive reactions had been caused by only BCG tuberculin, reactions to commercial tuberculin are also obtained in the majority of cases after a further few weeks. In guinea pigs infected with a virulent strain of the tubercle bacillus no difference can be demonstrated between the two types of tuberculin. Already in the first tests reactions are equally frequent to commercial tuberculin and to BCG tuberculin. In BCG-vaccinated newborn and older children, BCG tuberculin injected intradermally provokes reactions considerably more often than equivalent amounts of commercial tuberculin. The children sensitive only to BCG tuberculin in the first test after vaccination are subsequently sensitized to commercial tuberculin as well.