In four cases of sarcoidal and five cases of tuberculous lymphadenitis, the cellular composition of hypersensitivity-type granulomas was investigated by the application of monoclonal and polyclonal antibodies to cryostat sections. The composition of the granulomas was the same in the two disorders; they consisted of centrally placed OKM1 + OKIa + epithelioid histiocytes and multinucleated giant cells, together with few OKT4 + helper/inducer and many OKT8 + suppressor/cytotoxic T lymphocytes, some of which expressed Ia-like antigens. B lymphocytes were notably absent from this central part but formed a mantle around each granuloma and expressed predominantly or only IgD at their surfaces. Between the perigranulomatous cuff and the central epithelioid histiocytes, OKT8 + lymphocytes were distributed in a ring-like manner. The results suggest that the underlying immunologic mechanisms in sarcoidosis and tuberculosis are similar. Multinucleated giant cells may function as antigen-processing and -presenting accessory cells, and activated T cells, of either the helper or the suppressor type, may be operative in delayed-type hypersensitivity reactions. The presence of a predominance of IgD or of IgD alone at the surface of the perigranulomatous B lymphocytes may be related to a block in their terminal maturation stage, a defect in accessory cells, the presence of a local population of suppressor cells, or to the importance of IgD in granulomatous disorders.