The melanophore bioassay is a robust, sensitive, and versatile procedure for screening G protein-coupled receptors in a variety of formats. Because melanophores contain a wide variety of G proteins, they can be employed as a sensitive, real-time response system for studying transfected receptors and for defining equilibria for drug effects. This assay can be run in 96-well microtiter plates or in open-lawn 1536 format, and can yield conventional agonist-antagonist as well as constitutive assays.