We examined blood lymphocyte subpopulations in 20 patients with sarcoidosis, 37 patients with other diseases, and 51 normal subjects. The B-lymphocytes were identified by the presence of surface immunoglobulin or B-lymphocyte-associated antigen. Lymphocytes were also centrifuged with sheep erythrocytes for 5 min at room temperature at 200 g, and rosette formation was assayed immediately (active E-rosette-forming T-lymphocytes) or after 60-min incubation at 4 degrees C (total T-lymphocytes). The B-lymphocytes counts did not differ among the groups. The proportions of total E-rosette-forming T-lymphocytes and active E-rosette-forming T-lymphocytes were increased in the sarcoid patients, whereas absolute counts of both types of E-rosette-forming T-lymphocytes were not different from control counts. Active E-rosette-forming T-lymphocytes showed an inverse correlation with serum concentration of angiotensin-1-converting enzyme, a probable indicator of the disease activity. Incubation of normal lymphocytes with sarcoid plasma increased the proportion of active E-rosette-forming T-lymphocytes. This plasma rosette enhancement was correlated with the number of active E-rosette-forming T-lymphocytes in the blood from which the plasma was separated. These results suggest that a factor in sarcoid plasma affects the number of active E-rosette-forming T-lymphocytes and that high numbers of these cells are associated with disease stability.