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A simple method for the isolation of bovine retinal S-antigen by ion-exchange chromatography on DEAE Sephadex, alone or in combination with a chromatafocusing step is described. High yields of highly purified S-antigen were obtained. Analytical studies indicated that the isolated protein was a single chain, 55 000 Mr glycoprotein with little tendency to self-association and a pI of 5.5. S-antigen prepared in the absence of protease inhibitors migrated on SDS-PAGE as a doublet but close similarity between both protein bands was observed by analysis of papain digests, suggesting that the protein was readily susceptible to proteolysis. S-antigen prepared by this method induces a selectively posterior focal uveoretinitis in a dose-dependent manner.