The rheumatoid factor-like substance (RFLS) induced by hyperimmunization of rabbits with bacteria was consistently and exclusively associated with the macroglobulin fraction of the sera studied. The RFLS is separable from the bulk of serum proteins by zone centrifugation, DEAE cellulose chromatography, and gel filtration. Partially purified preparations of the RFLS were studied by immunoelectrophoretic analysis and analytical ultracentrifugation. Certain RFLS sera demonstrated selective reactivity with one or more rabbit isohemagglutinin antibodies. Pepsin treatment of rabbit γ-globulin removed or greatly diminishes its capacity to react with the RFLS.

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