Diffusion, which limits nutrient penetration and end-product export in biofilms, is restricted by reversible binding and extracellular volume fraction (Ve). Fluoride has been demonstrated to prevent calcium bridging, hence inhibiting calcium-mediated cell association (Rose, Lee and Shellis, Caries Res. 30 (1996) 458-464). 3H-inulin effusion measurements from streptococcal model plaques, at pH 7.0 or 5.0, 0-20 mmol/l Ca2+, and with or without 5 mmol/l KF, demonstrated that Ve was greatest in the absence of added Ca2+ and at pH 7.0, lowest at 20 mmol/l Ca2+ and pH 5.0, and that F- raised the minimum Ve. By bridging adjacent cells and reducing the net negative charge, calcium and low pH, respectively, reduce Ve. Fluoride eliminates the calcium-bridging effect, hence increasing Ve.