Circulating immune complexes were found in sera from patients with acute sarcoidosis during the initial phase of the disease. The symptoms of acute sarcoidosis may be explained by the presence of such complexes. The total number of peripheral T cells was lower in most patients with sarcoidosis than in healthy controls. The difference was most marked in cases with long duration of the disease. The composition of peripheral T cells was altered irrespective of the total number of such cells, and a lack of cells responding to PPD and con A was shown. Tis may be related to the impairment of delayed hypersensitivity reactions which may be present in all stages of the disease. The atypical mononuclear cells which have been found in blood from patients with sarcoidosis are lymphocytic cells. Some have B cell properties whereas some seem to have T cell origin. The presence of such cells may indicate a persistent activation of the immune system. Immunological reactivity determined by genes closely linked to the major histocompatibility complex does not seem to be responsible for susceptibility to sarcoidosis.