It is postulated that an initial step in dental plaque formation is the adherence of oral bacteria to the salivary pellicle. Recently, we have found that a proline-rich and basic glycoprotein (MGP) from human parotid saliva, which is successfully purified by Concanavalin A-Sepharose affinity chromatography, binds to some oral streptococci such as S. mitis and S. sanguis. This paper deals with some inhibitors which affect the binding of the MGP to S. sanguis ATCC 10557. The assay for the binding ability of the radioactive MGP to the bacterial cells was performed by incubation of the reaction mixture containing 10 microgram of [3H]MGP (6000 dpm) and about 4.5 mg of bacterial cells (dry weight) in 0.5 ml of 0.05 M Tris-HCl buffer containing 0.05 M NaCl, pH 8.0 with a final volume of 0.51 ml. After 1 hour standing at 4 degrees C, the cells were washed five times with the same buffer. The resulting sediment was solubilized in 0.5 ml of NCS tissue solubilizer and the radioactivity was measured. The binding of the radioactive MGP to bacterial cells was specifically inhibited by galactose, lactose and N-acetyllactosamine. Applications of heat and trypsin on the cell surface, strikingly reduced the binding ability. These findings strongly suggested that a lectin-like substance may be present on the bacterial cell surface.