Previous studies have indicated that a galactosephilic component present on the bacterial cell surfaces of Streptococcus sanguis ATCC 10557 may be responsible for the salivary glycoprotein-mediated binding of the cells. The purpose of this study was to investigate the purification and characterization of galactosephilic cell surface component from S. sanguis ATCC 10557. A galactosephilic component involving fibrils on the cell surfaces was isolated by the techniques of freezing and thawing, and purified by an affinity chromatography on beta-D-galactose binding-Bio-Gel P-2 followed by gel filtrations on Bio-Gel P-150 and on Bio-Gel P-30. Both disk gel electrophoresis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the purified product was homogeneous. The isoelectric point of the purified sample was 8.5 to 9.0. Treatment of the purified sample with pronase E reduced remarkably either the hemagglutinating activity or the precipitation reaction with proline-rich glycoprotein in human parotid saliva, suggesting that the active site may be present on the peptide moieties. When sugar specificity was examined by hemagglutination-inhibition test, D-galactose was the strongest inhibitor. The results of this study suggest that the galactosephilic component may be a bacterial lectin.