Rabbit thyroglobulin was partially digested with papain. Low molecular size fragments were separated by gel filtration and injected intravenously into rabbits. Circulating thyroid-specific antibodies appeared, as demonstrated by tanned cell hemagglutination, precipitation and passive cutaneous anaphylaxis. Lymphocytic and monocytic infiltrates were demonstrated in the rabbits' thyroid glands. Antisera to papain-treated thyroglobulin reacted more strongly with the papain-treated antigen, in

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... ast to rabbits immunized with untreated thyroid antigen. On analysis by precipitation in gel, the papain-treated preparation showed at least two additional precipitating components that gave reactions of partial identity in intact thyroglobulin. Papain-generated rabbit thyroglobulin fragments serve as effective immunogens in the same species following intravenous injection without adjuvant.