The HLA antigens in 40 patients with subacute thyroiditis were investigated. An association between subacute thyroiditis and HLA-Bw35 antigen as previously reported, was confirmed. Further immunological observations included a decrease in Serum Clq, C4 and C3 activator levels; conversely, there was an increase in serum C3, IgM, alpha1-acid glycoprotein and alpha1-antitrypsin levels. Serum IgA levels were decreased in those patients who were Bw35 negative, but were normal in the patients who were Bw-35 positive, with serum alpha2-macroglobulin levels the opposite was found. These studies support the view that a genetic factor(s), viz., the major histocompatibility system, plays a role in the pathogenesis of subacute thyroiditis. A "nosological clustering" around the Bw35 antigen as a marker for viral susceptibility is suggested.