The effect of highly purified rheumatoid factor on the precipitin reactions of various antigen-antibody systems was determined. The amount of nitrogen precipitated was increased over a broad range when the factor was added to ovalbumin, human albumin, or human gamma globulin, and the corresponding rabbit antibodies. In the zone of antigen excess, soluble antigen-antibody complexes were precipitated by rheumatoid factor. Soluble aggregates of human and rabbit gamma globulin, produced by heating at 63 degrees C., treatment with urea plus mercaptoethanol or treatment with guanidine, also precipitated with rheumatoid factor. Ultracentrifugal analysis of dissolved specific precipitates showed the presence of aggregated gamma globulin. The sedimentation rate of reactive aggregates was greater than 20 S, and concentrated preparations free of the non-reactive 7 S gamma globulin could be prepared by various procedures of zone centrifugation. These aggregates showed a high inhibitory capacity in the sensitized sheep cell agglutination reaction. Solid gamma globulin, prepared by heat denaturation, also selectively adsorbed the rheumatoid factor, and removed or decreased the activity in the various precipitation and agglutination reactions. Elution of highly purified active preparations from the solid gamma globulin could be carried out with urea or acid buffers. Evidence for interaction between rheumatoid factor and low molecular weight gamma globulin without precipitation, was also obtained. This interaction appears to occur in the circulation of patients with rheumatoid arthritis. The question of whether the rheumatoid factor represents an antibody to gamma globulin was discussed. Points of similarity to the behavior of complement also were cited.