Labial biopsy specimens from eight patients with primary Sjögren's syndrome (SS), 10 patients with secondary SS, and three healthy controls were studied with monoclonal antibodies identifying HLA class I and class II antigens; interferon-alpha, beta, and gamma; interleukin 2 (IL2); and the IL2 receptor (Tac) among others. In the normal biopsy specimens there was evidence of HLA class I and, to a lesser extent, class II antigens in both ducts and acini, though this was much less marked than in the Sjögren's biopsy specimens. Interferon-gamma staining, but not interferon-alpha or beta, was also considerably enhanced in the biopsy specimens from the patients with Sjögren's syndrome. These data support the view that in Sjögren's syndrome the release of interferon-gamma may be involved in the induction of class II determinants. Our observations were broadly similar in both primary and secondary Sjögren's syndrome except that patients with primary Sjögren's syndrome tended to have more diffusely scattered T lymphocytes.