Proliferating cells have been immunophenotypically characterized in lymph node and bronchoalveolar lavage (BAL) samples obtained from patients with active and inactive sarcoidosis with the cell-cycle-related antigen Ki67. Ki67 monoclonal antibody was used by combined immunohistochemical methods together with antibodies recognizing macrophage- and T-cell-subset-related antigens using avidin-biotin peroxidase (ABC) and alkaline phosphatase-anti-alkaline phosphatase (APAAP) systems. Many proliferating Ki67+ cells were found in affected mediastinal lymph nodes. These cells were mainly located around granulomas and exhibited phenotypical markers of helper/inducer T cells (CD3+, CD4+). Ki67+ macrophages could not be detected in the same lesions with this technique. A different picture was found in BAL preparations where proportions of both T lymphocytes and macrophages were Ki67+. The presence of replicating lymphocytes could be correlated to disease activity, whereas the proportions of Ki67+ macrophages did not show significant differences between active and inactive disease. Interleukin-1 (IL-1) expression was investigated in the same samples with a specific antiserum. Epithelioid macrophages in granulomas and BAL macrophages in all cases exhibited cytoplasmic staining revealing an activated status. Interestingly, giant cells in granulomas were mainly devoid of IL-1 immunoreactivity. These studies support the concept that activated cells at different sites of ongoing inflammation play a central role in the mechanisms accounting for granuloma formation.