Cayman's cGMP Assay is a competitive ELISA that can be used for quantification of cGMP directly obtained from cell lysates, tissue homogenates, plasma or urine. Since the antibody used in this assay was prepared against a cGMP-carrier protein conjugate, antibody binding is increased if an acetyl group is present on the 2' hydroxyl group of the cGMP. The optional acetylation procedure for both samples and standards increases the sensitivity of the assay approximately 10 fold.