A lab test that measures how strongly a compound grabs onto a receptor by seeing how well it displaces a reference molecule from that receptor.Produces Ki or IC50 values that quantify receptor binding affinity. In vitro receptor binding data characterizes the pharmacological interaction but does not predict in vivo efficacy or safety — a compound that binds tightly to a receptor in a test tube may still be metabolized before it reaches that receptor in vivo, or may produce different effects in a complex physiological system than it does in an isolated assay.