Bovine S-antigen was purified by gel filtration and ion exchange chromatography according to previously described techniques. An enzyme linked immunosorbent assay (ELISA) using antiserum to bovine S-antigen raised in guinea-pigs was employed to detect S-antigen in the chromatographic fractions. The purity of the S-antigen was determined by SDS-PAGE electrophoresis, where a single band was found. The purified S-antigen in microgram quantities together with Freund's complete adjuvant induced uveitis in rats two weeks after injection into the foot pad. Serum samples from children suffering from chronic uveitis and healthy children were tested for antibodies to S-antigen by the ELISA. A statistically significant difference in the level of specific antibodies between patients and controls was found. There was no clear-cut correlation between the severity of uveitis and antibody titre, but cases with retinal involvement and aggressive uveitis all showed definite elevations of antibodies to S-antigen.