The ability of isolated IgG and F(ab)2 fragments to agglutinate IgG-coated latex particles was used as a test of IgG-RF activity. No definite differences could be proved between controls and RF-negative RA patients, whereas the latex titres of IgG and F(ab)2 of sero-positive RA patients were higher. The IgG latex activity was recovered in the fractions between the 19S and 7S after gradient centrifugation of serum or isolated IgG at pH 7.2 and within the 7S fraction after centrifugation at pH 4.1. The dissociated IgG complexes reformed after neutralization, thus indicating a self-association of the IgG molecules.