Aims: To develop a simple and flexible novel in vitro model system to grow films of oral bacteria that could be used to study aspects of dental caries. Methods and Results: Standardized suspensions of bacteria were inoculated into Ultrafree-CL (Millipore) ultrafiltration units at various densities. These were incubated for varying time intervals with a range of carbon sources. The bacterial films reproducibly achieved between 10(7) and 10(8) cfu cm(-2), irrespective of the number inoculated and with no significant changes for 14 d. However, Streptococcus mutans grew through membranes with pores of diameter greater than 0.1 mum after 6 d. Culture of films in sucrose or water for 6 d led to a decreased number of colony-forming units, but returning them to broth reversed this. Conclusions: Reproducible films of oral bacteria can be cultured in Ultrafree-CL units. Significance and Impact of the Study: This study has shown that Ultrafree-CL units can be used as a simple model system to grow biofilms that could be used for dental caries research.