The mice were separated into four groups of five each. Three groups were infected intraperitoneally with 100 ml of a 16106 c.f.u. ml21 suspension of M. tuberculosis, M. bovis or M. canettii that had been cultured from soil inoculated for 12 months; one negative control group was inoculated with 100 ml sterile PBS. The weight, clinical examination and survival of mice were followed weekly for 90 days. All mice (all survivors) were killed by exsanguination at day 90 post-infection. The lungs, spleen and liver were removed. One portion of each organ was homogenized in