Aim: The aim of this study was to assess whether or not increasing levels of antibacterial polylysine (PLS) and monocalcium phosphate (MCPM) within dental composites can reduce Streptococcus mutans biofilm formation on surfaces. Methods: Five experimental composites were prepared with PLS (4,6,8 wt. %) and MCPM (8,12, 16 wt.%). Control samples were commercial materials Z250 (3M), Activa , Fuji II and the same experimental samples with no PLS or MCPM. Set discs (10mm diameter and 1mm depth) were immersed in 1 ml BHI broth with 1% (w/v) sucrose and 5*106 S. mutans then incubated in air with 5% CO2 for 24 hours. In each experiment, 3 discs with the same formula were tested 3 times (n=3x3). Biofilm thickness and mass on material surfaces were determined using confocal microscopy and crystal violet (CV) staining. Results: CV optical density decreased when MCPM and PLS concentrations were increased. Composite formula with MCPM 16 and PLS 8 showed the lowest level of bacterial reduction. Biofilm thickness was between 120 and 150 microns for all the commercial materials and the experimental control. With 4% and 8% PLS, biofilm thickness declined to 70 and 40 microns whilst 8% and 16% MCPM gave an average thickness of 60 and 50 micron, respectively. Biofilm mass decreased by 55% when PLS was increased from 4% to 8%, however, increasing MCPM from 8% to 16% did not affect the results. Conclusion: Doubling PLS content in composites caused a greater reduction in biofilm growth on dental composites than doubling MCPM.