The virulence for mice of some newly described slowly growing mycobacterial species, M. intermedium DSM 44049 (Group I), M. lentiflavum ATCC 51985 and M. interjectum ATCC 51457 (Group II), M. genavense ATCC 51233, M. celatum ATCC 51131, M. branderi ATCC 51788 and M. conspicuum DSM 6333 (Group III) was compared with the virulence of M. intracellulare N-260 (virulent strain). 10(6) CFU cultured in Middlebrook 7H9 medium were given i.v. to the BALB/c mice. Infected animals were sacrificed after one day and 56 days, and observed for the incidence and/or degree of gross lesions of visceral organs. The bacterial load (CFU) in tissue homogenates of the lungs and spleen was counted using 7H11 agar plates. Macroscopically, marked enlargement of the spleen and numerous small whitish nodules in the lungs of mice infected with M. intracellulare were noted, whereas with other test strains no gross disease in the lungs and slight enlargement of the spleen of mice infected with M. genavense and M. branderi were noted. The average log CFU in the lungs and spleen 56 days after the infection was in the order as follows: M. intracellulare (5.90 and 7.57, respectively), M. genavense (4.35, 7.15), M. celatum (5.01, 6.00), M. branderi (4.09, 6.33), M. conspicuum (3.11, 5.10), M. intermedium (2.26, 4.19), M. lentiflavum (1.70, 3.91), and M. interjectum (1.43, 2.93). Microscopically, granulomatous lesions mainly consisted of macrophages with acid-fast bacilli were found in the alveolar walls and splenic red pulp of mice infected with M. intracellulare. No histopathological changes were produced in the lungs of any of mice infected with other test strains, and some milder features were revealed in the spleen of mice infected with M. genavense, M. branderi and M. celatum. Thus, we concluded that all of the test strains of the newly described nontuberculous slowly growing mycobacteria were less virulent for mice than were M. intracellulare.