We previously reported that alveolar lymophocytes in patients with active sarcoidosis are sensitized Propionibacterium acnes (P. acnes) which may play a significant role in the induction of alveolitis in these patients. We further investigated the production of Interleukin-2 (IL-2), and the responsiveness to IL-2 of alveolar lymphocytes obtained from sarcoidosis patients and stimulated by P. acnes in vitro. In 21 untreated sarcoidosis patients, 7 treated patients and 13 control subjects, the mean IL-2 activity of fluid released from cultured alveoloar lymphocytes was 9.8±15.7 (M±SD), 1.9±4.7 and 0.2±0.8 u/ml respectively. The IL-2 activity of lymphocytes from untreated patients was significantly higher than that of control subjects (p<0.02). Neither perioheral lymphocytes in sarcoidosis patients nor in controls produced IL-2 stimulated by P. acnes. The responsiveness of alveloar lymphocytes to recombinant IL-2 was evaluated by 3H-thymidine uptake in the presence and absence of P. acnes. Lymphocytes stimulated by P. acnes showed a significantly increased uptake (3,766±3,929 dpm) compred to ustimulated lymphocytes (1,123±968 dpm) obtained from 11 untreated sarcoidosis patients (p<0.02). On the other hand, the responsiveness of lymphocytes obtained from 6 control subjects was low regardless of stimulation by P. acnes. There was a significant correlation (p<0.05) between the P. acnes-induced production of IL-2 by alveolar lymphocytes and the blastogenesis of alveloar lymphocytes in untreated sarcoidosis patietnes. Our findings indicate that P. acnes stimulated IL-2 production and IL-2 receptor induction in alveolar lymphocytes from patients with active sarcoidosis. This study supported our hypothesis that P. acnes could be an antigen causing sarcoidosis.