We report video microscopy measurements and inverse simulation analyses of specific Ca2+-dependent interactions between N-cadherin fragments attached to supported lipid bilayer-coated silica colloids in quasi-2D concentrated configurations. Our results include characterization of the bilayer formation and fluidity and the attachment of active extracellular cadherin fragments on bilayers. Direct measurements of interaction potentials show nonspecific macromolecular repulsion between cadherin fragments in the absence of Ca2+ and irreversible bilayer fusion via cadherin-mediated attraction at > 100 mu M Ca2+. Analysis of Ca2+-dependent N-cadherin bond formation in quasi-2D concentrated configurations using inverse Monte Carlo and Brownian Dynamics simulations show measurable attraction starting at 0.1 mu M Ca2+, a concentration significantly below previously reported values.