The interaction of salivary lysozyme with the surface protein antigen (PAc) of Streptococcus mutans and the interaction of lysozyme with the pathogen were examined by ELISA using S. mutans MT8148 (PAc+) and the PAc-defective mutant EM-2 (PAc-). The lysozyme clearly bound to the S. mutans wild type but not to the S. mutans mutant. Furthermore, lysozyme bound directly in the fluid phase to the rPAc, of which the binding kinetics were determined (K-m = 3.63 +- 0.04 times 10-3 M-1 s-1, K-off = 1.72 +- 0.04 times 10-5 s-1 and K-on/K-off = 2.11 times 10-8 M-1) using surface plasmon resonance. The kinetics of both association and dissociation were relatively slow. In addition, anti-lysozyme antibody significantly inhibited the binding of salivary components to the rPAc. The present findings indicate that lysozyme is one of the major salivary components interacting with S. mutans PAc.