Abstract A study has been made of inhibition of homologous and heterologous precipitin reactions in Types III and VIII antipneumococcal horse and rabbit sera using, as inhibitors, a series of oligosaccharides of varying chain length, isolated from acid and enzymatic hydrolysates of S3 and S8 by gel filtration. Up to 6–8 hexose units, the longer the chain length of an oligosaccharide, the more effective it becomes as an inhibitor of the homologous precipitin reaction. Cross reactions are more readily inhibited than homologous reactions and relatively low concentrations of tetrasaccharides or even disaccharides prevent the precipitation of cross-reacting antigens containing disaccharide units that are also present in the homologous polysaccharide. The presence of glucuronate residues in a ligand is of particular importance in determining its effectiveness as an inhibitor. Certain anomalies encountered in interpreting results using ligands containing four or more hexose units, can be explained by assuming that the inhibitors involved may be divalent.