G protein-coupled receptors (GPCRs) are of great importance to the pharmaceutical industry as they are involved in many human diseases and include well-validated targets for therapeutic intervention. Discovery of lead compounds, including small synthetic molecules, that specifically inhibit the receptor's function, is an important initial step in drug development and relies on sensitive, specific, and robust cell-based assays. Here, we describe a kinetic cellular assay with a fluorescent readout primarily designed to identify receptor-specific antagonists that inhibit the intracellular Ca release evoked upon the activation of the CXC chemokine receptor 4 (CXCR4) by its endogenous ligand, the CXC chemokine ligand 12 (CXCL12). A key advantage of this method is that it also enables screening of compounds endowed with intrinsic agonistic properties (i.e., compounds eliciting an increase in intracellular Ca concentration in the absence of CXCL12) or compounds modulating the receptor's function via interaction with allosteric binding sites (i.e., positive and negative allosteric modulators (PAMs and NAMs, respectively)). On the down side, autofluorescent compounds might interfere with the assay's readout, thereby hampering reliable data interpretation. Most likely this assay can be implemented, with minimal adaptations, as a generic drug discovery assay for many other GPCRs of which the activation leads to a release of intracellular Ca.