All so-called immunizing agents (both living and dead tubercle bacilli) have failed to produce dependable immunity in laboratory and domestic animals. During the first quarter of the twentieth century numerous prominent scientists and clinicians re-emphasized that dependable immunity to tuberculosis does not exist. Later enthusiasm for BCG resulted from insufficient periods of observation, comparing unlike conditions, disregarding other factors, etc. No one knows what present day so-called BCG cultures contain. They are not composed of a single bacterial form as Calmette isolated, but of multiple bacterial forms which vary markedly in invasiveness of animal tissues. No two cultures now in use are alike. So-called BCG has recently caused progressive and fatal disease in laboratory animals in certain physiologic and pathologic states as well as in normal golden hamsters. Not infrequently it produces clinical tuberculosis in humans, at least at the sites of administraton and in regional lymph nodes.