The concentrations of immunoglobulins IgG, IgM and IgA and their age dependence were determined in the serum of normal, untreated beagle dogs by a newly developed sandwich enzyme-linkedimmuno-sorbent-assay (ELISA). A clear age-dependent increase between « 0.8 and 1.6 years of age was observed for the immunoglobulin IgA, whereas IgG and IgM showed only a slight tendency to an agedependent increase. For immunotoxicological characterization of various compounds especially in long term studies, this IgA age-dependence has to be considered in the planning and interpretation of studies with beagle dogs.