Summary1. Sarcoidosis is characterized by granuloma formation in many tissues. The origin of the granuloma formation remains unknown.2. The Kveim reaction (intradermic injection of a suspension prepared from sarcoid tissue) is a reliable aid in the diagnosis of sarcoidosis when a validated suspension is used. The reaction seems to be the expression of a specific lymphocyte response to an antigen of the suspension.3. The number of circulating lymphocytes is decreased in the active forms.4. Many parameters of T-cell function point to a decreased cellular immunity (impaired delayed hypersensitivity skin reactions, low E-rosette, impaired antigenic and mitogenic stimulation).5. Antilymphocytic factors can be demonstrated in sarcoidosis serum and are perhaps responsible for the depression of T-cell function.6. Other findings suggest a concomittant T-cell stimulation (atypical lymphocytes, increased spontaneous DNA formation and MIF production, Kveim reaction as an expression of cellular immunity).7. The level of immunoglobins is significantly increased. There is a raised antibody titer against several viruses. This is interpreted as a stimulation of the humoral system.8. Circulating immunecomplexes have been demonstrated in a limited number of cases.9. The possible role of lymphocyte interactions and feedback mechanisms in the phenomena observed is discussed.