Increased production of tumor necrosis factor alpha (TNF-) appears to play an important role in the progression of human immunodeficiency virus disease. One treatment strategy being explored is the use of TNF- inhibitors. TNF- also appears to be important in conferring resistance to infections, and the inhibition of this cytokine may exacerbate the emergence of opportunistic pathogens, such as Mycobacterium avium complex (MAC). The present study examines the possibility that inhibition of TNF- will increase the progression of disease in mice infected with MAC. C57BL/6 beige (bg/bg) mice have been shown to be highly susceptible to infection with MAC and are routinely used for testing of antimycobacterial drugs. However, bg/bg mice are known to exhibit impaired phagocyte and natural killer cell function. Since these cell types are important sources of TNF-, the susceptibility of the bg/bg strain to infection with MAC was compared with those of the heterozygous (bg/+) and wild-type (+/+) strains of C57BL/6 mice. The susceptibilities of the bg/bg and bg/+ strains of mice infected with MAC were found to be comparable. The +/+ strain was the least susceptible. Mycobacterial burden and serum TNF- levels increased over time in all the strains of mice tested. The bg/+ strain of C57BL/6 mice was then chosen to measure the activity of TNF- antagonists. Treatment with dexamethasone decreased serum TNF- levels and increased mycobacterial burden. Treatment with anti-TNF- antibody or pentoxifylline did not significantly alter serum TNF- levels but increased mycobacterial burden. Treatment with thalidomide neither consistently altered mycobacterial burden in the spleens or livers of infected mice nor affected serum TNF- levels.