A radioimmunoassay was developed for detection of immunoglobulin in the aqueous of normal horses and horses with intraocular diseases. Levels of albumin were detected by radial immunodiffusion. Results of assays on samples from normal eyes from which aqueous was obtained by paracentesis under anesthesia were 32.10±21.50µg/ml for IgG, 0.05±0.01µg/ml for IgM, 0.04±0.02µg/ml for IgA and 34.0±38.0µg/ml for albumin. Results in 138 normal eyes sampled post mortem were 41.56±38.65µg/ml for IgG, 0.18±0.43µg/ml for IgM, 0.46±1.45µg/ml for IgE and 184±240µg/ml for albumin. Levels in abnormal eyes were some 50–120% greater, and very high levels were noted in cases of clinically documented uveitis.The aqueous to serum ratio of the proteins measured generally varied in relation to the molecular weights and was increased in diseased eyes. The ratio of IgG/albumin in the diseased eyes was less which suggested leakage of protein from an impaired blood/aqueous barrier rather than intraocular antibody synthesis.