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Ricinus agglutinin purified to homogeneity precipitates certain serum glyco-proteins containing terminal non-reducing galactose residues. Immunoelec-trophoresis of human sera in agarose gel with ricinus agglutinin in the antibody trough gave 2 fusing precipitin lines due to reaction with IgM and haptoglobin. All of 50 monoclonal IgM proteins precipitated with ricinus agglutinin, and the reactive sites were localized to the Fc μ fragment. By affinity chromatography on Sepharose columns containing insolubilized ricinus agglutinin prealbumin, albumin, α1-lipoprotein, α1-antitrypsin, α1-acid glycoprotein, α1-antichymotrypsin, α2HS glycoprotein, Ge-globulin, caerulo-plasmin, β-lipoprotein, β1 C-globulin, and transferrin were not retained, whereas α2-macroglobulin, haptoglobin, IgM, IgA, and IgG were retained and could be eluted with lactose. Fused rocket immunoelectrophoresis revealed varying degrees of microheterogeneity among the latter proteins; almost all of IgM but only a small fraction of polyclonal IgG was retained on the ricinus column.