The antibodies to bovine S-Ag in sera were determined by ELISA in 43 patients with acute or chronic, anterior or panuveitis and in 32 healthy persons as controls. The results showed that the antibodies titers to bovine S-Ag in the patients with panuveitis were not different from those in the controls (P greater than 0.05). However the antibody titers to bovine S-Ag in the patients with anterior uveitis were significantly lower than those in the controls (0.01 less than P less than 0.05). It is suggested that there are naturally occurring antibodies to S-Ag in health persons. The relationship between antibodies to S-Ag and uveitis is discussed. It is indicated that natural antibodies to S-Ag may play a protective role in the healthy subjects.