Circulating alpha-interferon in plasma of 26 patients with Sjögren's syndrome was 0.069 +/- 0.034 ng/ml, a significant decrease compared with 0.119 +/- 0.051 ng/ml for age- and sex-matched healthy subjects (P less than 0.01) and compared with values previously found for healthy donors at ages 1-89 years. The results indicate the inability of Sjögren's syndrome patients to maintain circulating alpha-interferon.