Summary: A specific method for purification of cross-reacting antibodies from equine anti-pneumococcal sera of types III and VIII has been described. Quantitative studies with mixtures of S3 and S8 have shown that although cross-reacting antibodies isolated from each serum type may be completely precipitated by either polysaccharide alone, the homologous antigen is selectively precipitated from mixtures of the two. An octasaccharide has been isolated from enzymatic digests of S8. Following reduction of the terminal unsaturated hexuronic acid residue with tritium gas, the labeled octasaccharide has been used as a ligand in equilibrium dialysis against type VIII antibodies isolated from horse and rabbit sera. Binding curves show homogeneous affinity at all ligand concentrations.