We evaluated the ability of a dual-species community of oral bacteria to produce the universal signalling molecule, autoinducer-2 (AI-2), in saliva-fed biofilms. Streptococcus oralis 34, S. oralis 34 luxS mutant and Actinomyces naeslundii T14V were grown as single- and dual-species biofilms within sorbarods fed with 25% human saliva. AI-2 concentration in biofilm effluents was determined by the Vibrio harveyi BB170 bioluminescence assay. After homogenizing the sorbarods to release biofilm cells, cell numbers were determined by fluorometric analysis of fluorescent antibody-labelled cells. After 48 h, dual-species biofilm communities of interdigitated S. oralis 34 and A. naeslundii T14V contained 3.2 x 10(9) cells: fivefold more than single-species biofilms. However, these 48-h dual-species biofilms exhibited the lowest concentration ratio of AI-2 to cell density. Oral bacteria produce AI-2 in saliva-fed biofilms. The decrease of more than 10-fold in concentration ratio seen between 1 and 48 h in S. oralis 34-A. naeslundii T14V biofilms suggests that peak production of AI-2 occurs early and is followed by a very low steady-state level. High oral bacterial biofilm densities may be achieved by inter-species AI-2 signalling. We propose that low concentrations of AI-2 contribute to the establishment of oral commensal biofilm communities.