Streptococcus mutans plays an important role in the formation of dental plaque. To study biofilm growth on hydroxyapatite (HA) in vitro, a flow system based on a Modified Robbins Device (MRD) and a method for the quantification of the biomass using fluorescent staining with SYTO(R) 9 were developed. The combined approach was used to assess the inhibitory effect of plant extracts on biofilm formation in concentrations below their minimal inhibitory concentrations.