1. The products from papain and pepsin hydrolyses of the guinea-pig immunoglobulins γ1G and γ2G were isolated and characterized with regard to molecular weight, amino acid composition, hexose content and antigenic specificity. 2. Fragments Fab and (Fab′)2 from immunoglobulins γ1G and γ2G have similar electrophoretic and antigenic properties, but show some class-specific differences in amino acid composition. 3. Three Fc fragments were obtained after papain digestion of immunoglobulin γ2G, namely, fragment Fc dimer (mol.wt. 58000), fragment Fc monomer (mol.wt. 29000) and fragment Fc′ (mol.wt. 8000). A single crystalline fragment, namely fragment Fc′ (mol.wt. 11000), was isolated after papain digestion of immunoglobulin γ1G. 4. Peptic digestion of immunoglobulins γ1G and γ2G releases C-terminal fragments, namely, fragments pFc′, of similar molecular weight (13000) but different amino acid compositions and distinct antigenic specificities. 5. Digestion-time studies show that immunoglobulin γ1G is far more susceptible to proteolysis than is immunoglobulin γ2G and suggest that at least a proportion of molecules are split primarily at a site that liberates fragment γ1Fc′.