Identification of individual cyclic AMP binding proteins in tumor cytosolic extracts is possibly owing to the ability of 32 P-labeled 8-azido cyclic AMP to act as an effective analog of cAMP, to bind specifically to its protein effector sites, and on photo activation to incorporate covalently to these sites by means of a highly reactive nitrene derivative. Resolution of labeled proteins can be achieved by SDS-PAGE and visualization by autoradiography (1 ).