Susceptible BALB/c mice were infected with Mycobacterium avium TMC 702. Groups of mice were then infused with 10(4) U (approximately 400 U/h) of murine beta interferon (IFN-beta) via a minipump system, and the progression of the infection was assessed. Mice infused with IFN-beta showed superior resistance to infection, as determined by reduced bacterial growth in the livers and spleens of infected animals, (1-log reduction in bacterial CFU at 2 months postinfection; P less than 0.001). This was corroborated by the fact that resident peritoneal macrophages treated with IFN-beta in vitro (10(2) U/ml) were more bacteriostatic for M. avium TMC 702 than their untreated counterparts. Overall, these findings suggest an important role for IFN-beta in mycobacterial infections.