PathHunter β-Arrestin Assays are homogeneous cell-based microplate assays that directly measure GPCR activation without a second messenger, imaging, or reporter gene. The assays measure arrestin interaction with the activated GPCR, detecting it via EFC-based chemiluminescence. The technology is nearly universal for GPCRs, and large assay windows and simple assay protocols make it attractive for screening G-coupled receptors.