This protocol assays inhibition of in vivo binding of [3H]-cAMP to cAR1 by GTPγS. Dictyostelium discoideum respond to extracellular cAMP through the cAMP chemoattractant receptor (cAR1). Binding of cAMP to the G protein-coupled cAR1 is inhibited by the GTP analog GTPγS. Protocol includes information on: Solutions used, BioReagents and Chemicals and Protocol Hints.