The tuberculous granuloma, induced by injection of microgram doses of killed mycobacteria into guinea pigs sensitized by injection of Freund adjuvant is immunologically mediated. Formation of the granuloma is preceded by development within 24 h of a lymphocyte-dominated mononuclear cell response typical of a delayed hypersensitivity (type IV immune response) reaction. About the sixth day, following a marked decrease in intensity of the cellular reaction, a nodule containing monocytes and macrophages develops at the injection site. With increasing numbers of monocytes and macrophages the nodule forms a non-caseating granuloma with giant cells but dominated by epithelioid cells and reaching a maximum size about 3 wk after injection. Thereafter the granuloma undergoes gradual demolition being replaced and surrounded by fibroblasts and collagen deposition. The very delayed nature of this immune response as well as its histological character appear clearly to separate it from classical cell-mediated and humoral immune responses. These facts justify the hypothesis of a third type of (usually protective) immune response characterized histologically by the development of an epithelioid cell granuloma and determined by the nature of the antigenic material and the reactivity of the host. The initial polymorphonuclear leucocyte reaction to injection of mycobacteria, being similar in sensitized and control animals, does not appear to be under immunological control.