Total and fractionated proteins were evaluated in the serum of normal penguins. Four proteins with antibody activity, determined by passive haemagglutination, were isolated by Sephadex G-200 filtration and DEAE-cellulose chromatography, from the serum of penguins (Pygoscelis adeliae) inoculated with dinitrophenylated human gamma-globulin. Immunoelectrophoresis and immunodiffusion showed that all these proteins exhibited both their own and shared antigenic determinants. On account of their electrophoretic mobility, elution conditions and characteristics of the precipitation arcs, they have been tentatively denominated IgG2 (gamma2), IgG1 (gamma1), IgM (gammam) and IgX (probably IgA).