Purpose: To assess the antibacterial effect of orthodontic cements containing 1% insoluble antibacterial polycat-ionic nanoparticles against Streptococcus mutans. Materials and Methods: Polycationic polyethyleneimine (PEI)-based nanoparticles were incorporated into GC Fuji Ortho LC (GC), CX-plus (Shofu) orthodontic cements and in Neobond Transbond plus (Denstply) and Transbond XT (3M) orthodontic adhesives. The samples were evaluated immediately after setting, as well as after two weeks and one month of aging. The antibacterial effect against S. mutans was evaluated with the direct contact test and the agar diffusion test. In addition, the antibacterial properties of the eluate from the examined materials was tested. Four-way ANOVA followed by Tukey's test was used to determine bacterial growth rates. Results: S. mutans outgrowth was substantially reduced (p 0.05) following direct contact with the surface of Neobond adhesives (95%, i.e. 5-6 log reduction) and GC Fuji Ortho LC cement samples (97% reduction) containing PEI nanoparticles. CX-plus cement, Transbond plus and Transbond XT adhesives with and without PEI showed no antibacterial effect, and S. mutans outgrowth was similar to that of the controls. Conclusions: Neobond adhesive and GC Fuji Ortho LC cement with 1% incorporated insoluble antibacterial polycat-ionic nanoparticles exhibited stable antibacterial properties, particularly after immediate contact between the cement and the adhesive, and thus may prevent S. mutans outgrowth adjacent to orthodontic appliances. Further studies are needed to evaluate the efficacy, physical properties and possible side effects of the PEI nanoparticles in vivo. Keywords: antibacterial, direct contact test, orthodontic cement, quaternary ammonium polyethyleneimine, S. mutans