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S U M M A R YIn an attempt to locate the antigens associated with protection against foot-rot the fine structure of Fusiformis nodosus and the interaction between the organism and specific antiserum obtained from vaccinated animals were studied.The high titres of circulating agglutinin associated with immunity were shown to be due to the interaction between antibodies and pili. These observations are discussed in relation to the structure and pathogenicity of other Gram-negative organisms. I N T R O D U C T I O NOvine foot-rot is a highly contagious disease characterized by inflammation of the interdigital skin and hoof matrix leading to separation of the hoof from the soft tissues. The disease is due to a mixed bacterial infection of the uncornified epidermis by Fusiformis nodosus and F. necrophorus (Beveridge, 1941 ;Egerton, Roberts & Parsonson, 1969;. Prolonged wet conditions are necessary to allow bacterial invasion of the skin; the spread of foot-rot ceases when the environment becomes dry.Foot-rot is a chronic disease and repeated infections confer little, if any, natural immunity. Successful protection of sheep against artificial challenge with foot-rot following subcutaneous or intramuscular inoculation of a vaccine consisting of a formolized suspension of Fusiformis nodosus emulsified in an oily adjuvant was reported by Egerton & Roberts (1971). Similar vaccines were also effective under conditions of natural challenge (Egerton & Burrell, 1970;Skerman, 1971). Good field protection has also been reported for a vaccine with potash alum as adjuvant (Roberts, Foster, Kerry & Calder, 1972).In an attempt to locate the antigens associated with the stimulation of immunity and to elucidate the effects of antibody on the bacterial cells the ultrastructure of Fusidormis nodosus and the interaction between F. nodosus cells and specific antiserum obtained from vaccinated animals were studied. METHODSOrganism. Fus$ormis nodosus cultures, Wellcome Research Laboratories collection 6475 and 6476 were used.Growth. Portions (450 ml) of the culture medium consisting of 3 % bacteriological peptone (Wellcome Reagents Ltd) +o-5 % yeast extract (Difco) were inoculated with 50 ml of an overnight broth culture. Samples were removed at appropriate intervals, measured for turbidity in a nephelometer and examined with the electron microscope.

SUMMARY An account is given of the life cycle of Dermatophilus dermatonomus as observed in cultures and in lesions of mycotic dermatitis. Factors which were shown to govern the stages of development in cultures, appear to have a similar influence within the lesion. It is suggested that the sheep's skin may be the only natural habitat for D dermatonomus and that survival of the species may therefore depend on the repeated transmission of infection.