Objective: To study the effect of specific anti-S. mutans egg-yolk immunoglobulin G (IgY) on S. mutans and artificial dental caries in vitro. Methods: Streptococcus mutans B29-33, an over-expression GTF strain, was used as antigen, and 25 hens were immunized. IgY was isolated from immunized hens’ egg yolks. The growth and the adherence of S. mutans MT8148 were observed after it was incubated with or without IgY. The quantities of demineralization were measured after the enamel blocks were incubated with or without IgY during the period of artificial dental caries formation. Results: High titers of immunoglobulin antibodies specific to S. mutans were found in both chicken serum and egg yolk after series immunization. IgY was shown to aggregate S. mutans significantly, and the sucrose-dependent adherence of S. mutans to a glass surface decreased with the increase in IgY concentration. Furthermore, the amount of enamel demineralization decreased in artificial dental caries when IgY was used. Conclusion: Specific anti-S. mutans IgY can be obtained from immunized hens’ egg yolks. IgY can aggregate S. mutansand inhibit the sucrose-dependent adherence of S. mutans to a glass surface. It also can decrease the demineralization of enamel during the period of artificial dental caries formation.