To examine the immunologic alterations in patients with sarcoidosis we characterized the cell cycle phase of T lymphocytes that were collected from peripheral blood and from bronchoalveolar lavage fluid. T-cell DNA and RNA contents were measured at the single cell level using flow cytometry after staining with the metachromatic fluorochrome acridine orange. T-enriched lymphocyte suspensions were obtained from peripheral blood and from bronchoalveolar lavage in 17 patients with histologically-proved sarcoidosis (10 patients, stage I and 7 patients, stage II) and in 4 patients with acute extrinsic hypersensitivity pneumonitis (AEHP). The percentages of cells in the S + (GS + M) phase in the peripheral blood of the patients with sarcoidosis did not differ from those of healthy control subjects. With bronchoalveolar lavage, however, elevated numbers of T cells in the S + (G2 + M) phase were found in the patients with AEHP and in those with stage II sarcoidosis when compared to patients with stage I sarcoidosis. The cellular RNA content of T lymphocytes from the peripheral blood showed a typical bimodal distribution without difference between patients and control subjects. Conversely, T lymphocytes obtained by bronchoalveolar lavage from patients with AEHP and sarcoidosis had a homogeneous low RNA content which differed from that of T lymphocytes from the blood from that of in vitro phytohemagglutinin-stimulated lymphocytes. These findings provide a new approach to the study of the mechanisms of local T-cell activation in sarcoidosis.