Previous studies showed that the high minimum infective dose (more than 10(6) organisms) of biovar A strains of Fusobacterium necrophorum for mice by subcutaneous inoculation could be greatly reduced, often to less than 10 organisms, by suspending the fusobacteria in sublethal doses of broth cultures of certain other bacterial species, such as Staphylococcus aureus. The present study showed that no such enhancement of infectivity occurred with two biovar B strains. This observation, together with the low pathogenicity of pure cultures of these strains for mice, suggests that biovar B plays no more than a minor role in the aetiology of necrobacillosis.