SUMMARY: A simple assay has been developed to quantify the cAMP-relay in Dictyostelium discoideum. The assay is based on the stimulation of cells, in the presence of a phosphodiesterase inhibitor, with 2-deoxyadenosine 3,5-monophosphate (dcAMP) at a concentration which saturates cell surface cAMP receptors. The accumulation of cAMP is measured by an isotope dilution assay using a cAMP-binding protein with such a low affinity for dcAMP that purification of cAMP can be omitted.