Severe nontuberculous mycobacterial infections in patients who are not infected with HIV have been shown to be due to abnormalities in the pathways that generate or use interferon gamma (IFN gamma). In some of these patients treatment with IFN gamma has been effective in improving or curing these infections. Recently, interleukin-12 (IL-12) has been shown to be a potent inducer of IFN gamma along with other cytokines. Experiments in animals and preliminary experience by us in humans suggests that IL-12 may be an important adjunct to antimycobacterial therapy. We seek to use IL-12 in a phase I/II trial in the treatment of severe nontuberculous mycobacterial infections in patients who have not been cured by the best tolerated conventional therapy with IFN gamma. Patients will be studied for inborn or acquired immune defects as well as IL-12 responsiveness in vitro under protocol 93-I-0119 "Detection and Characterization of Host Defense Defects". Patients will receive IL-12 subcutaneously 2 times weekly. We will use an intrapatient dose escalation protocol ranging from 20 ng/kg to 300 ng/kg, depending on the highest dose tolerated by the patient. We expect this study to yield valuable information about tolerance and toxicity. We seek to treat 10 patients over the next 3 years.