In human sera showing elevated levels of IgM, the serum agglutinators may be of both IgG and IgM classes. When the quantity of IgG is very low, these antibodies may be solely of the IgM class. Elevations in IgM levels subsequent to loss of IgG through the kidney, as in nephrosis. and elevations in all immunoglobulin levels, as observed in severe suppurative infection, do not alter the production of serum agglutinators as IgG globulins.