A number of novel biologic agents are being introduced to replace, enhance, or modulate immune responses in medical illnesses. The use of these therapies has become crucial in treating some of these diseases, yet there is relatively little available information about their cost-effectiveness. Two examples are presented. Interferon gamma, used in chronic granulomatous disease, costs about $140 for a 100-microg vial; yearly costs average $21840 per patient. Study data estimated a 69% to 76% reduction in serious illness with interferon gamma treatment; a reduced incidence of infections could cover drug costs. Intravenous immunoglobulin is used lifelong in antibody deficiency and clearly reduces the number of serious illnesses. Projected savings derive from fewer hospital admissions and reduced organ damage, but infusion costs vary widely because of the prices charged for the drug and infusion services.