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Immunization of HSA-tolerant animals with sulphanil-HSA induces the formation of antibodies that react with native HSA. To test whether the anti-HSA response does actually represent complete restoration of the original reactivity, the antibodies produced following immunization of tolerant animals with sulphanil-HSAS were compared to those produced by normal rabbits immunized with the native antigen. Gel-diffusion analysis of the precipitins disclosed that the patterns of the two types of antibody were not completely identical. The antibodies of the 'restored' tolerant rabbits were directed mainly towards an antigenic determinant of the native HSA, to which normal rabbits immunized with HSA did not respond. When the 'restored' rabbits were challenged with native HSA after a prolonged rest period, the antibodies formed thereafter were completely identical with anti-HSA produced by normal HSA-immunized rabbits. Thus, complete restoration of the original immunological reactivity to HSA was achieved following an intermediary stage of atypical anti-HSA elicited by the conjugated protein. Attempts to break down natural tolerance to RSA by treatment with sulphanil-RSA failed to give any evidence of an autoimmune elimination of RSA. To test whether this inability to break tolerance to RSA was due to the presence of an excess of native protein, animals that had been made tolerant to HSA were immunized with a mixture of sulphanil-HSA and native HSA. Under such conditions, the termination of acquired tolerance was inhibited. The possible relevance of these observations to the cellular basis of immunological tolerance is discussed. [ABSTRACT FROM AUTHOR]