Römer (1903) has demonstrated that white mice injected intraperitoneally with large doses of tubercle bacilli isolated from man survived longer than mice injected with tubercle bacilli isolated from cattle. The blood of the spontaneously dead animals contained large numbers of tubercle bacilli. In the present study, red mice are injected intraperitoneally with 10 mg doses of different species of mycobacteria, and the number of bacilli in the blood is estimated at various intervals within the first 24 hours after the inoculation. The number of bacteria is considerably higher in the blood of mice injected with M. bovis, but, in contrast to M. bovis, M. avium disappears rapidly from the blood stream. Supplementary experiments show that red mice injected with M. bovis have a shorter survival time than mice injected with M. tuberculosis, and that the bacteraemia induced by M. bovis into white mice is clearly less pronounced than in red mice.