Bovine strains of Mycobacterium tuberculosis were isolated from 137 of 5021 cases of tuberculosis (2.7%) occurring in South East England during the years 1977 to 1979. These were divisible into 'classical' and 'Afro-Asian' bovine strains according to their susceptibility to pyrazinamide. Classical strains were the predominant type among the Europeans and Afro-Asian types were commoner in immigrants. Bovine strains caused a high incidence of extrapulmonary disease in both ethnic groups and there was a significant relationship between renal disease and 'classical' strains. The age and sex distributions of bovine strains were similar to those of human strains in both ethnic groups. It was considered that these bovine strains were transmitted from human to human by the pulmonary route.