The Cayman Chemical cGMP Assay is a competitive ELISA that can be used for quantification of cGMP directly obtained from cell lysates, tissue homogenates, plasma, or urine. The assay has a range from 0.23-30 pmol/ml and a sensitivity (80% B/B0) of approximately 1 pmol/ml. 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. A protocol for acetylating both the standards and samples prior to performing the assay is provided. Basal levels of cGMP in cell lysates can often be measured without acetylation, but results will depend on the type and number of cells being utilized. Platelets produce approximately 1.5-2.5 pmol cGMP/109 platelets under basal conditions. Cells such as NG108-15 cells and monocytes produce considerably more cGMP than platelets (approximately 0.1-1 pmol/106 cells).