Retinal S-antigen (S-ag) was purified by monoclonal antibody (MoAb) immunoaffinity chromatography from soluble protein extracts of bovine and human retina. Purification of S-ag was readily achieved by affinity chromatography using four different MoAb-Sepharose 4B columns. The four antibody columns gave different recoveries with material of comparable enrichment with greater than 95% purity as judged by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The use of two different MoAbs covalently bound to Sepharose 4B and known to be directed to disparate, spacially distant epitopes on S-ag led to at least a twofold increase in recovery, with the aforementioned purity. Immunoaffinity purified S-ag retained its serological and uveitogenic properties. The high recovery of S-ag associated with this one-step procedure is preferable to conventional preparatory techniques, and enables high antigen recovery when tissue availability is limited (eg human retina).