A study was made of the immunologic properties of the A and B fractions of human and guinea pig γ2-globulins obtained by reduction and alkylation in aqueous medium and separation by Sephadex in acid, using rabbit anti-γ-globulin antisera. In both cases the B fraction, containing the L chains, could be shown to possess only part of the antigenic determinants of the native γ-globulin, while the A fraction showed a reaction of complete identity with the parent molecule. Digestion by papain of the A fraction yielded three main fragments, one of which, SA, was shown to be immunologically identical to the S fragment obtained by papain digestion of the γ-globulin molecules. From 50 to 60% of the proteins of the A fraction from human γ-globulin could be specifically precipitated by a mixture of rabbit antisera against Bence-Jones protein types I and II. The possible reasons for the presence of large amounts of L chain antigenic determinants in the A fraction are discussed.

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This study was supported by the United States Public Health Service, Grants AI-04983, A2594 and 1431, by the Health Research Council of the City of New York under Contracts I-138 and I-274 and by the Fundação de Amparo à Pesquisa, São Paulo, Brazil.

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