Much work on bacterial adhesion has focussed on the colonisation of surfaces and the removal of mature biofilms. Little attention has been devoted to interactions within mature biofilms and how these might be manipulated in the cause of novel therapies. Calcium binding to oral streptococci displays characteristics of positive cooperativity and calcium uptake does not follow the same mechanism as calcium release [Rose et al., J. Dent. Res. 72 (1993) 78-84]. An investigation of the variation in dissociation constants found with respect to cell concentration (from 2.77+/-0.66 mmol/l at 5 g/l [cell] to 1.28+/-0.37 mmol/l at 20 g/l), and in particular the non-zero value of the apparent binding constant (K(app)) on extrapolation to zero cell concentration, revealed that calcium uptake could be explained by a ligand-facilitated mechanism of cell association. Hence, cell association follows a route that starts with essentially irreversible long-range interactions between the cells, mediated by as yet unidentified macromolecules, followed by reversible calcium bridging. This suggests that cells are held in place within biofilms by a web of polymers, but that proximity to neighbouring cells is dependent on calcium bridging and that this may be manipulated to allow increased penetration of therapeutic agents.