In a field experiment on irrigated pasture, sheep of several breeds were vaccinated twice, subcutaneously, in the upper neck, with Bacteroides nodosus vaccine containing either depiliated cells (DC vaccine), or whole, piliated cells (WC vaccine) and the responses were measured over the following 14 weeks. DC vaccine was as effective as WC vaccine in protecting against the development of foot-rot under conditions of moderate challenge, although the WC vaccine induced significantly higher pilus agglutinating antibody titres. Foot-rot developed in significantly more vaccinated Merinos (Peppin and Saxon strains) than in Romney Marsh, Dorset Horn or Border Leicester breeds. Agglutinating antibody titres after WC vaccination were significantly lower in the Peppin Merino than in the other sheep for the first 6 weeks, while after DC vaccination the titres remained elevated longer in the Border Leicester and Saxon Merino and were significantly higher from 6 weeks onwards. Reactions at the inoculation sites were generally larger in the British breeds than in the Merinos and among the former the reactions were largest, most numerous and most frequently discharged their contents in the Dorset Horn. Bodyweight gains in all vaccinated sheep were initially reduced, compared with controls, but the differences were no longer significant after the eighth week.