Two methods for provoking a haemagglutinating macroglobulin identical or similar to the Rheumatoid factor (RF) are described. The macroglobulins were obtained (a) by injections of Diplostreptococcus agalactiae into animals, mainly white rats; ( b ) by injecting into rats the antibody to RF (anti-RF) or the active fraction of this antibody. The anti-RF was produced by immunizing rabbits against RF. Both (a) and ( b ) have given rise to haemagglutinating macroglobulins showing high titres (up to 1 : 1024) in the sheep cell test. The titre is much more stable when using method (a). Using method (b) the results show sudden variations with sometimes total disappearence of the haemagglutinating macroglobulin.The anti-RF serum produced neutralizes the RF completely in precipitation tests. The active principle of anti-RF is a fraction showing a sedimentation constant of about 7 S, i.e. exactly the same as that of the common yG. Comparing the 7 S, which has the property of neutralizing RF completely in unheated state, and the common yG (IgG) by immunoelectrophoresis, the first does

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