1. A series of experiments is described in whichthe efficacy of vaccination with the vole acid-fast bacillus against tuberculosis in cattle has been studied. Various methods of vaccination have been tried and intravenous inoculation has proved to be the most satisfactory.

2. A single intravenous inoculation of 5 mg. of vole bacilli can raise the resistance of cattle to infection with the bovine bacillus to a high degree, this resistance being evidenced by a complete absence of infection or by a marked diminution in the severity of lesions following exposure to artificial or natural infection.

3. The resistance takes time to develop, reaches its maximum between 6 and 18 months after vaccination and then slowly wanes. The resistance induced by a single inoculation is of insufficient duration in many animals to warrant the practice of single vaccination being used in the field except in special circumstances.

4. Revaccination can intensify resistance provided that it is delayed until the end of the fourth week. A long-term experiment is being carried out to determine if revaccination, in addition to intensifying the initial resistance, can also lengthen its duration.

5. The degree of resistance set up in cattle is correlated with the virulence of the vaccinating strain of vole bacillus for its natural host. This fact will necessitate that any strain used for vaccine purposes will have to be maintained at a high level of virulence by repeated passage through its native host.

6. Care is necessary in the selection of a strain of the vole bacillus for vaccine purposes, for we have encountered one strain, G564, which can set up a fatal infection in calves. Nothing has been noted which would contra-indicate the use of strain LV 285.

7. The histopathological changes pathognomic of tuberculous infection which follow the intravenous inoculation of vole bacilli are mainly confined to the lungs and thoracic glands. They reach their maximum about the 28th day and have undergone resolution by the 84th day.

8. The changes which occur in the cells of the reticulo-endothelial system following vaccination have been described. These changes must be taken into consideration in deciding the optimal time and dosage for revaccination and a further study of them is advisable.

Our thanks are due to Prof. T. Dalling for his unfailing interest in our work and for much helpful advice, and also io Dr A. Q. Wells for suppfyihg cultures of vole bacilli and for the loan of his multiple-puncture instrument. We are indebted to Dr V. D. Davies for painstaking examination of the joint lesions and to Dr J. R. M. Innes, who examined many histological sections of vole-inoculated animals and drew up a short report of which we have made much use. Mr V. L. Mastin was associated with us during part of the work and we gratefully acknowledge the help he gave. We also owe a deep debt of gratitude to our technical assistants, particularly Mr W. Nicholson and Mr C. Spurgeon, much of the routine bacteriological work being done by the former whilst the latter prepared the bulk of the histological sections. The local slaughterhouse officials of the Ministry of Food, particuiarfy Mr Bacchus, the slaughterhouse manager, and Mr C. Rose, the foreman slaughterman, have been most co-operative over the arrangements for the slaughter of many animals. Our thanks are also due to the farmers who allowed us to expose experimental animals to natural infection on their farms, and to these and other farmers who have placed their herds at our disposal for the purpose of field trials.

The cost of these investigations was met mainly by grants from the Agricultural Research Council, but partly from the funds of the Institute of Animal Pathology.