Chronic beryllium disease is characterized by the accumulation of helper/inducer T cells, macrophages, and granulomas in the lungs. To evaluate the hypothesis that the proliferation of CD4+ (helper/inducer) T cells in the lungs of patients with this disorder is maintained by local activation of beryllium-specific T-cell clones, we studied T cells obtained from peripheral blood and by bronchoalveolar lavage in eight patients and five healthy controls. The proliferation of T cells in response to beryllium in vitro was confined to the CD4+ T cells from the patients and was dependent on the presentation of antigen in the presence of both major histocompatibility complex class II antigens and functional interleukin-2 receptors. T cells from the patients' lungs had a significantly greater response to beryllium than did T cells from their peripheral blood (stimulation index, 103 vs. 5; P less than 0.01). Lines and clones of cells developed from T cells from the patients' lungs showed dose-dependent proliferation in response to beryllium but did not respond to recall antigens or to other metals. Although all beryllium-specific T-cell clones were CD4+ and none were CD8+ (suppressor/cytotoxic), all beryllium-specific clones studied had different rearrangements of T-cell antigen receptors, suggesting that the response to beryllium involved T cells with diverse specificities for beryllium. We conclude that in patients with chronic beryllium disease, beryllium acts as a class II-restricted antigen, stimulating local proliferation and accumulation in the lung of beryllium-specific CD4+ (helper/inducer) T cells. Hence, chronic beryllium disease is a hypersensitivity disease in which beryllium is the specific antigen.