Disseminated infection with Mycobacteria avium complex (MAC) is one of the most common systemic bacterial infections in advanced stages of the acquired immunodeficiency syndrome (AIDS). Current therapy for disseminated MAC infection in HIV patients consists of multidrug chemotherapy regimens are often accompanied by toxicities, and many patients become intolerant of one or more agents. Macrolides are the essential component of successful therapy, yet macrolide resistant strains are being recognized with increasing frequency. Thus, there is an interest in identifying additional therapeutic interventions for disseminated MAC in HIV-infected patients. Interleukin-12 (IL-12) is a central, regulatory cytokine in cell-mediated immunity. IL-12 enhances the cytolytic activity of cytotoxic T and NK cells, and induces interferon-gamma (IFN gamma) production from T and NK cells. This open-label Phase I study is designed to evaluate the safety and immunologic/microbiologic effects of interleukin-12 administration in HIV-infected patients with concomitant disseminated Mycobacterium avium (MAC) infection. Fifteen patients with documented disseminated MAC will be randomized to receive double-blinded placebo or escalating doses of IL-12 in addition to anti-MAC chemotherapy and standard anti-retroviral therapy for six weeks. IL-12 will be administered subcutaneously, with escalating doses every month over the dose range of 30 ng/kg, 100 ng/kg, and 300 ng/kg, or until an individual maximum tolerated dose (IMTD) is reached. Should a patient receive 2 consecutive blood cultures negative for MAC during the course of the study at a lower dose, then he/she will not be further dose escalated. Those patients receiving placebo after 6 weeks will be crossed over to receive the full treatment course of IL-12. Each new dose or dose escalation will take place on an inpatient basis. Once a patient is clinically stable at a dose, the patient will be maintained at that dose as an outpatient for the remainder of the month. Total IL-12 administration will not exceed 12 weeks, or 24 total doses.