Immunoglobulin G (IgG) in aqueous humor from patients with various uveitis syndromes was analyzed using a number of immunologic techniques. Sixty-five percent of patients with Fuchs' heterochromic cyclitis (FHC), 70% of patients with other forms of uveitis, and 44% of controls showed local synthesis of IgG, as demonstrated by an elevated IgG:albumin relative concentration ratio. Using an enzyme-linked immunosorbent assay to measure the concentration of IgG subclasses 1-4, a relative excess of IgG1 was found in the aqueous compared with the serum in FHC. Isoelectric focusing and immunoblotting studies revealed oligoclonal IgG bands in the aqueous of 13 of 23 (57%) patients with FHC, most being of the IgG1 subclass. Oligoclonal bands were not found in 18 patients with other types of uveitis or 13 patients undergoing surgery for senile cataract. These findings indicate intraocular production of IgG of restricted specificity in FHC, providing further evidence for local immune dysfunction in this condition. As yet the antigenic stimulus for this oligoclonal B-cell response has not been identified.