Calcium bound by oral bacteria may have important effects on plaque cohesion and mineral dynamics. Equilibrium dialysis was used here to study the effect of magnesium and zinc competition on calcium-binding by Streptococcus sanguis 7863. Experiments were carried out using 45Ca in 0.1–5.0 mmol/l Ca2+ carrier. Increasing [Mg] and [Zn] decreased the apparent Ca-binding affinity, allowing calculation of the affinities forMg2+ andZn2+. Dissociation constants were found to be 0.94 ± 0.23, 1.32+0.39 and 1.26+0.41 mmol/l (whole cells) and 1.87+0.49, 3.05+1.24 and 4.08 ± 1.29mmol/l (cell walls), for Ca, Mg and Zn respectively. Binding capacities for Ca (and, by inference, Mg and Zn) were 27.7 ± 2.8 μmol/g wet weight (whole cells) and 54.2+1.6 μmol/g wet weight (cell walls). These data suggest that Mg and Zn act as true competitive inhibitors of calcium binding.