We have recently identified a novel transcript of the CTLA-4 gene that may represent a native soluble form of CTLA-4 (sCTLA-4). To determine whether sCTLA-4 was expressed in humans, we applied a sensitive enzyme immunoassay on serum from patients with autoimmune thyroid disease (ATD). Eleven of 20 patients with ATD had circulating levels of sCTLA-4 ranging from 28 to 78 ng/ml, whereas only 1 of 30 apparently healthy volunteers had a level greater than 4 ng/ml. sCTLA-4 immunoreactivity was inhibited by its binding to B7.1, suggesting that sCTLA-4 is a functional receptor. Immunoprecipitation analysis of serum from patients with ATD revealed a polypeptide consistent with the predicted size of sCTLA-4. We conclude that a native soluble form of CTLA-4 is derived from an alternate transcript of the CTLA-4 gene, and its level in plasma is elevated among a population of patients with ATD.