Anti-Rh agglutinins were inactivated by peroxidase in the presence of H2O2. This inactivation was by peroxidative oxidation and did not depend necessarily on the presence of any low molecular weight dialysable factor. Uric acid does not enhance the effect of peroxidase but may actually interfere with the inactivation. Peroxidative inactivation of anti-Rh agglutinins has been correlated with a decrease in ratio of maximum to minimum absorption in the ultraviolet spectrum of the oxidized serum proteins, manometric evidence of oxidation of human globulin containing anti-Rh agglutinins, and microbiological assay evidence of a decrease of tyrosine groups after treatment of the serum proteins with peroxidase plus H2O2. Although this evidence is not considered conclusive, and the effect is not specific for Rh antibodies, it is suggested that the biological activity of the anti-Rh agglutinin may require intact and unaltered tyrosine moieties in the protein antibody.