Foot-rot in sheep has troubled farmers for many years not because of any spectacular epidemics associated with high mortality but because the chronic lameness associated with foot infection, and its consequences, frequently interfere with farm husbandry. Methods of control and eradication have been known since 1940 but as they generally involve many hours of strenuous work, the development of more efficient methods of foot-rot control has been the aim of different research groups. Two findings which suggested that foot-rot organisms might be accessible to blood-borne inhibitors helped change the attitude of researchers involved with this problem. The first was the successful use of parenterally administered antibiotics against foot-rot infection and the second was the demonstration that serum globulins were able to make contact with Fusiformis nodosus Fusiformis nodosus was shown to be the primarily important and transmitting organism of sheep foot-rot. in situ. The results of these experiments carried out at the McMaster Institute, Sydney led to a reappraisal of existing research projects and the formation of a new policy leading towards the development of experimental vaccines. By 1970 when the writer's course of study began, it had been shown that bench scale vaccines conferred some protection against foot-rot infection and that vaccination was effective in stimulating resistance even after infection had become established. There were no methods available other than direct sheep challenge experiments for evaluating different types of vaccine and although the existence of anti-F. nodosus bactericidal antibody was known, there was still considerable doubt about the immune mechanisms involved in protection. There had been no investigation of the antigenic potential of F. nodosus products of growth or indeed of the bacterial cells themselves and as a consequence the influence of media components on the immunogenicity of the resulting culture had received little attention.