BALB/c and severe combined immunodeficient (SCID) mice were inoculated intraperitoneally with Mycobacterium avium and the numbers of cfu were monitored for 70 days in spleen, liver, lung, kidney, brain and peritoneum. While BALB/c mice formed typical granulomas and controlled bacterial growth in organs, a delay in development of lesions and a modest containment of infection were observed in SCID mice. In the spleen of BALB/c mice, in which bacterial growth was contained, macrophages (Mo) and natural killer (NK) cell numbers increased > or = 4.2 times and T- and B-cell numbers increased > or = 1.8 times after 42 days of infection; conversely, a low recruitment of mononuclear cells was observed in the spleen of SCID mice, where M. avium proliferated efficiently. Unlike visceral organs, a pronounced decrease in the number of cfu was observed in the peritoneum of BALB/c mice, concomitantly with a > or = 31.7-fold increase in Mo and NK cells and a > or = 9.1-fold increase in T and B cells. In the peritoneum of SCID mice only a bacteriostatic effect was observed despite a > or = 56.7-fold increase in Mo and NK cells and a > or = 22.3-fold increase in T and B cells. These results suggest that while an intact immune response can efficiently control M. avium infection in the spleen and peritoneum of BALB/c mice, cells of the innate immune system such as Mo and NK cells play a role in the containment of bacterial growth in the peritoneum, but not spleen, of SCID mice.