Hyperimmunization of two groups of rabbits with killed Escherichia coli and Bacillus subtilis has resulted in the formation of a serum factor (RFLS) which resembled the human rheumatoid factor. It was a heat-stable protein that migrated electrophoretically in the gamma-beta globulin range and sedimented rapidly with ultracentrifugation. The serologic properties of the RFLS were destroyed by mercaptoethanol. It reacted primarily with rabbit gamma globulin in an aggregated state (immune complexes or physically aggregated gamma globulin) and demonstrated cross-reactivity with human gamma globulin.

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