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In Denmark and probably in other countries as well, the infectious agent causing intercurrent death of rabbits by passages of Nichols pathogenic Treponema pallidum has been studied in rabbits in the absence of T. pallidum. This agent can be propagated in rabbits at intervals of 2 to 30 days and, depending on the interval between passages and the number of passages, the mortality may vary from zero to almost 70 per cent. Based on the post mortem findings in fatal cases, the name pleural effusion disease is suggested for this rabbit infection. Iridocyclitis, haematological and biochemical changes are signs of the disease not described previously. The source of the infectious agent is discussed.

Groups of red mice were injected intravenously with doses of from 3 viable units to 3times107 viable units of a virulent strain of M. bovis grown in Dubos Tween medium and exposed to ultrasonics. The course of the infection was evaluated by the survival times of animals that had died spontaneously and by quantitative culture from the organs of animals injected with the four smallest doses and killed at various times after the injection. M. bovis provoked progressive, fatal infection in all animals, even in those injected with 3 viable units. The median survival times of the animals injected with the three largest doses were 13, 17 and 18 days, i.e. increasing the lower the dose. For the animals injected with the three smallest doses, the median survival times were all about 3 months, independent of dosage. On autopsy, tuberculous lesions were found in spleen, lungs and lymph glands, but never in the liver. These lesions, which rapidly became caseous, were largest in the animals injected with a small dose, particularly in those which survived the longest. Smears from the organs contained enormous numbers of tubercle bacilli. Quantitative culture from the organs showed that the tubercle bacilli multiplied strongly during the first 3–4 weeks after the injection. Multiplication then ceased or became strongly inhibited. During the following 3 weeks, the number of viable units in the various organs remained at about the same level, being greatest in the spleen, smaller in the liver and lymph glands, and smallest in the lungs. During the rest of the experimental period, the number varied considerably from animal to animal, but generally it was almost unchanged in the lymph glands, liver and spieen, but increasing in the lungs. In some of the animals that lived longer than 3 months, the number of viable units in all the organs was extremely great.

The homology percentages between DNA from M. smegmatis ATCC 607 and DNA from nine various species of mycobacteria have been determined. DNA-DNA hybridization was measured in a spectrophotometer. The technique, calculation of results and the uncertainty of the method are described.