A simple procedure is described for measuring cyclic AMP (cAMP) in plasma, body fluids, and tissue extracts. A modification of the method of Brown et al. [Biochem. J. 121, 561 (1971)], it is based on competitive binding of cAMP by a protein in crude adrenal extracts, and precipitation of the cAMP-protein complex by ammonium sulfate. cAMP concentrations in normal human plasma ranged from 8 to 16 nmol/liter, and in the plasma of dogs from 5 to 15 nmol/liter. The range of the method is 0.5-8 pmol per assay, and up to 0.4 ml of sample can be used. A cheap and easily prepared binding-protein is used and the procedure is suitable for clinical studies of cAMP concentrations in plasma.