Three immunoglobulin classes (IgA, IgM, and IgG) have been isolated and characterized from serum pools of the Northern Fur Seal. These classes share antigenic determinants with human IgM and IgA, and porcine IgG. Two subclasses of IgG were identified based on DEAE anion exchangers. Quantitative serum levels of the immunoglobulins have been determined in animals ranging from full term pups to adults in addition to colostrum from mothers to evaluate the routes of passive immunity. Data indicate in utero synthesis of IgM, poor gut absorption of IgA and selective absorption of one subclass of IgG. The immune competence of the fur seals will be discussed.