Current methods for screening cell receptor internalization often require complex image analysis with limited sensitivity. Here we describe a novel bioassay based on detection of changes in global fluorescence lifetime above a gold substrate, with superresolution axial sensitivity and no need for image analysis. We show that the lifetime of enhanced green fluorescent protein expressed in a cellular membrane is greatly reduced in close proximity to the gold, resulting in a distance-dependent lifetime distribution throughout the cell. We demonstrate the application of this phenomenon in a screening assay by comparing the efficacies of two small molecule inhibitors interfering with the internalization process of a G protein-coupled receptor.