Protective immunity to intracellular bacteria such as mycobacteria and salmonella depends on intact cell-mediated immunity. Major effector mechanisms of cell-mediated immunity involve activation of macrophages by T helper-1 cytokines, particularly interferon-gamma. Patients with genetic deficiency of T helper-1 cytokines (IFN-gamma, IL-12) or T helper-1 cytokine receptors (IFN-gamma receptor, IL-12 receptor) are susceptible to infections with poorly pathogenic mycobacteria, and salmonella, suggesting that T helper-1 cytokines are essential in host defense against these pathogens. This review reports on the genetic and clinical characteristics of primary deficiencies of interferon-gamma activation pathways.