1. African pouched rats (Cricetomys gambianus) are very suitable animals for plague experiments on account of their tolerance of captivity and their extreme susceptibility. After inoculation with small doses of plague culture of ordinary virulence they always die.

2. Broth-grown vaccine was much more efficient as a prophylactic than agar-grown vaccine. When two doses were given a survival rate of 56 per cent, was obtained with the former and 25 per cent, with the latter.

3. A vaccine composed of carbolised spleen pulp from animals, which had died of acute plague, was rather more efficient than broth vaccine, giving a survival rate of 75 per cent. But it is not likely to be of practical value, as preparation is difficult, there are undesirable local effects and its superiority over broth vaccine is not very great. Its efficiency is attributed to the fact that the organisms which it contains are a true “body strain.” The method of preparation is similar to that of Hindle's yellow fever prophylactic.

4. A number of experiments are described in which virulence was abolished or reduced by passage through immune or partially immune rats. This occurred only when the passage animal showed a considerable degree of immunity, as indicated by the length of the survival period after the test infection. Cultures made from the abscess at the site of infection were less often attenuated than those from the liver of an immunised animal, but this was observed only in the case of large well-defined abscesses. It is suggested that a firm abscess wall protects organisms inside it against the action of antibodies appearing in the general circulation.

A culture attenuated by passage through an immunised rat was restored to normal virulence by successive passages through normal rats, but the process was slower than in the case of cultures attenuated by long cultivation, five passages being required in the former case and only one in the latter.

5. Cultures used in the later work, unlike those used in the earlier work, lost virulence rapidly when kept at room temperature. Experiments and observations are described which suggest that old strains, even when subjected to rat passage, are less stable as regards virulence than those recently isolated from human epidemic cases, and that the high temperature of the Gold Coast has an adverse effect upon virulence.

6. Attempts to isolate stable variants of B. pestis were not successful. Colonies vary much in form. Three types of colony are mentioned, but all intermediate grades were observed. Certain old avirulent strains had a greater tendency to produce large irregular colonies than recent strains, but the relation was so loose that the form of colony gave little indication of the degree of virulence. In old avirulent strains the bacilli tend to become longer and filaments of great length may be present. Salt stability tends to become less as a strain grows old and avirulent, but the change is not very marked and is apt to be masked by individual fluctuations.