Organisms of the Mycobacterium avium complex are associated with disseminated infection in patients with AIDS. The mechanisms that account for the survival of the intracellular bacteria are unknown. We document here that infection of C57BL/6 black mice with M. avium 101 triggered interleukin-10 (IL-10) production. The synthesis of IL-10 peaked after 2 weeks of infection and remained elevated throughout the period of infection. Treatment of M. avium-infected peritoneal macrophages with recombinant IL-10 suppressed the stimulatory effect of tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor. To confirm the possible role of IL-10 in the infection in vivo, mice were infected with M. avium 101 and simultaneously received treatment with neutralizing anti-IL-10 antibody. After 4 weeks the animals were harvested and the numbers of viable bacteria were quantitated in the liver, spleen, and blood. The liver and spleen of animals receiving anti-IL-10 antibody had 2 to 3 log units fewer bacteria than did those of control animals. These results suggest a role for IL-10 in the pathogenesis of M. avium infection.