The following thesis consists of twelve papers recording the results of an investigation concerning various aspects of brucellosis. With one exception all have been published, and this paper has now also been accepted for publication. The work described has proceeded over a period, of ten years from 1943 to 1953 at the Agricultural Research Council's Field Station, Compton, Berkshire, the main object being; the furtherance of knowledge pertaining to the immunisation of cattle against brucellosis. The technique of experimentation in cattle on a scale calculated to be sufficiently large to provide reliable results has been developed, particular attention being directed towards ensuring that the previous relevant history of the experimental animals was known. Special isolation precautions have been evolved to ensure that no extraneous infection could gain access to the animals during the experimental period and, to enable comparisons to be made between different immunising antigens, a standardised method of infecting the experimental cattle with brucellosis has now been established. Much of the information so far obtained, has already been utilised by the Ministry of Agriculture and Public Health authorities in this and other countries to assist in formulating policies designed to prevent the occurrence of brucellosis in cattle and in man. It was inevitable that certain results had to be published in conjunction with one or more colleagues because this field of research was too vast for any one individual to cover alone. Accordingly, in order to maintain continuity, these four additional papers have been incorporated in the text in sequence, rather than in an appendix. In these conjoint experiments the present author has been wholly responsible for all aspects of the work directly concerning the experimental cattle and in addition has performed all the serological and many of the cultural and biological tests.