Alfred G. Gilman developed a simple and sensitive assay for adenosine 3′:5′-cyclic monophosphate (cAMP) based on the competition for protein binding. The assay uses a cAMP-dependent protein kinase to form a nucleotide-protein complex, which is adsorbed onto a cellulose ester filter. The binding constant is approximately 10−9 M, allowing the assay to detect as little as 0.05–0.10 pmol of cAMP. The method is highly sensitive and accurate, making it suitable for estimating cAMP levels in various tissues. The assay conditions are such that cAMP destruction is not a factor, and the procedure is straightforward to perform. The author recommends routine use of the assay, despite the crude nature of the protein kinase preparation, due to its high sensitivity and ease of use.Alfred G. Gilman developed a simple and sensitive assay for adenosine 3′:5′-cyclic monophosphate (cAMP) based on the competition for protein binding. The assay uses a cAMP-dependent protein kinase to form a nucleotide-protein complex, which is adsorbed onto a cellulose ester filter. The binding constant is approximately 10−9 M, allowing the assay to detect as little as 0.05–0.10 pmol of cAMP. The method is highly sensitive and accurate, making it suitable for estimating cAMP levels in various tissues. The assay conditions are such that cAMP destruction is not a factor, and the procedure is straightforward to perform. The author recommends routine use of the assay, despite the crude nature of the protein kinase preparation, due to its high sensitivity and ease of use.