The aim of the present study was to investigate whether the level of selenium and selenium/vitamin E supply influences the humoral immunity of rats. In order to detect the effect of Se supply and age, 36 weaned Sprague-Dawley rats divided into two equal groups were killed after 22 or 45 experimental days by decapitation (Exp. I). In Exp. II 9 groups of 10 rats each were exposed to each combination of deficient, normal or excessive selenium with a vitamin E supply and killed after 44 days. The basic (deficiency) diet which was the same in both experiments contained 0.04mg Se and 8mg vitamin E per kg dry matter. The supplementation per kg diet was 0 or 0.2mg Se and 30mg vitamin E in Exp. I and 0, 0.2 or 1mg Se and 0, 30 or 200mg vitamin E in Exp. II. The concentration of selenium in serum, liver and spleen samples and the activity of glutathione peroxidase, which were determined to define the selenium status of the animals, corresponded well to the required supply situation. The immunoglobulins of type IgA, IgM and IgG with the subtypes IgG1, IgG2a, IgG2b and IgG2c were measured by immunoelectrophoresis. In both experiments selenium deficiency decreased the values of the IgG groups only nominally, IgA was not changed. IgM was significantly reduced, especially with prolonged selenium deficiency and simultaneous vitamin E deficiency. An excessive selenium supply compensated to a great extent for the effects of vitamin E deficiency on IgG and IgA.