Syngeneic mice of the C3H/He strain have been inoculated with cells of the plasmocellular tumours X5563 and X5647, producing myeloma proteins of the IgG and IgA class respectively, and were immunized with (1) sheep red blood cells, (2) bovine serum albumin and (3) horseradish peroxydase. The immune responses of the various groups of mice were followed. The antibody activities towards all three antigens were decreased in mice carrying the IgG producing tumour X5563. This effect was correlated with an enhanced catabolism of IgG as well as with a decrease in number of IgG antibody producing cells. A reduced immune response was also observed in mice to which the IgA producing tumour X5647 had been transferred, but the effect was significantly less pronounced. There was no effect at all observed on the IgM response. The avidity which was measured for the various IgG antibodies was not modified by the presence of IgG nor IgA secreting myeloma tumours.