C-polysaccharide (PnC) is the major surface component of pneumococci containing phosphoryl choline residues. In order to investigate the possibility that PnC can bind to glycolipid receptors present on epithelial cells we extracted carbohydrate material from a nonencapsulated strain of pneumococci. The components of the extract were separated by gel permeation chromatography. An ELISA was used for detection of fractions binding to the pneumococcal glycolipid receptor asialo GM1. These fractions were pooled and analysed by nuclear magnetic resonance spectroscopy (NMR). The 1H NMR spectrum showed good agreement with a reference spectrum of pure PnC showing that this substance was the major component. Binding of the purified PnC to asialo-GM1 was unaffected by protease K treatment. Immunoblots of the purified PnC after separation by SDS-PAGE resulted in a characteristic banding pattern. PnC could be released from pneumococci by heat treatment of whole bacteria in buffer as shown by reaction with a monoclonal antibody specific for the phosphoryl choline determinant. After separation by SDS-PAGE of the components of the heat extract, immunoblots showed the presence of bands characteristic for PnC. Eluates from the characteristic bands in the gel were shown to contain material binding to asialo-GM1. This binding was not reduced upon treatment with protease K. Pneumococci deprived of choline by cultivation in a medium containing ethanolamine as the only amino alcohol source did not bind to asialo-GM1, indicating that the phosphoryl choline residue of PnC is essential for the interaction between PnC and the glycolipid receptor. These data provide evidence that PnC containing an intact phosphoryl choline residue is a ligand responsible for binding of pneumococci to the receptor asialo-GM1.