Aqueous humour from 23 patients with Fuchs' heterochromic cyclitis (FHC) was analysed by a number of immunological methods. Intraocular IgG synthesis was found in 65% of patients and oligoclonal IgG bands, mainly of the IgG1 subclass, identified in 57%. There was a relative increase in IgG1 (P<0.01) as compared to patients with senile cataract. Local production of the cytokine Interleukin-6 was demonstrated in 63% of patients (P<0.01). Analysis of aqueous by HPLC and SDS-PAGE failed to reveal any abnormalities specific for FHC.These findings add further evidence to the theory of immune dysregulation in this condition.