Summary: Modifications of 2 hemagglutination procedures—the SEA and FII tests—for the identification of the rheumatoid agglutinating factors are described. Fractions II, III, IV-1, IV-4 and V were prepared by existing ethanol fractionation procedures from sera of patients with rheumatoid and non-rheumatoid diseases. Only Fraction III prepared from sera of patients with active peripheral rheumatoid arthritis and psoriatic arthritis produced positive reactions with the SEA and FII tests. The rheumatoid factors were not demonstrated in the serum or its fractions from patients with other types of rheumatoid and non-rheumatoid diseases. The electrophoretic characteristics of Fraction III were not indicative of the presence of the rheumatoid agglutinating factors. The iso-antibodies and the rheumatoid factors present in Fraction III were shown by absorption procedures to be qualitatively distinct immunological reactants. By absorption and cross reaction procedures, we established that the reactants in the SEA and FII tests were heterogenetically related. Variations in the reactivity of rheumatoid sera as well as their derived Fraction III for the SEA and FII reagents were observed. Several aspects of the hemagglutination reactions are discussed.