Equilibrium fluctuation analysis of single binding events has been used to extract binding kinetics of ligand interactions with cell-membrane bound receptors. Time-dependent total internal reflection fluorescence (TIRF) imaging was used to extract residence-time statistics of fluorescently stained liposomes derived directly from cell membranes upon their binding to surface-immobilized antibody fragments. The dissociation rate constants for two pharmaceutical relevant antibodies directed against different B-cell expressed membrane proteins was clearly discriminated, and the affinity of the interaction could be determined by inhibiting the interaction with increasing concentrations of soluble antibodies. The single-molecule sensitivity made the analysis possible without overexpressed membrane proteins, which makes the assay attractive in early drug-screening applications.