Two related species of the genus Ceratophrys (C. ornata and C. cranwelli) were studied by immunological techniques in order to find non-morphological characters which would allow to classify them more precisely. Bidimensional immunodiffusion of serum proteins showed extensive crossreactivity between both species, thus revealing their close relatíonship. However, by immunoelectrophoretic techniques, differences between their serum fractions were detected. The antisera raised in rabbits against the serum proteins showed different potency in reacting with the sera of each species. Sharper precipitation bands were obtained with the homologous serum. Two lipoproteic fractions, in the serum of both species, were also characterized.