Interferon-gamma is one of the cytokines which have various immunoregulatory functions. In the present study, the serum interferon-gamma level was determined in autoimmune diseases. It was increased in the active cases of systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD). Since there was a positive correlation between the serum interferon-gamma level and the rate of peripheral mononuclear cells positive for HLA-DR antigen in systemic lupus erythematosus, circulating interferon-gamma might have a biological functions as suggested by many in vitro studies. In rheumatoid arthritis and Sjögren's syndrome, there was no correlation between the serum interferon-gamma level and the clinical findings. These data suggest that interferon-gamma might be associated with the pathogenesis of autoimmune diseases such as SLE and MCTD, and it can be one of the indices for their disease activity.