Cattle produced humoral mouse-protective antibody (MPA) to Mycoplasma mycoides var mycoides as a result of natural infection, hyperimmunisation, vaccination with well-known strains of living vaccines or vaccination with killed cultures. Bovine MPA was distinct from complement-fixing and precipitating antibodies and was frequently detected in their absence, particularly in the sera of vaccinated--as opposed to naturally infected--cattle. In view of its defensive role in artificially-infected mice, it seems likely that such antibody plays a part in resistance to the disease in cattle. High levels of MPA followed inoculation of cattle with killed concentrated culture containing adjuvant; this finding merits further investigation, as it may lead to improved methods of vaccination.