A series of systematically modified cyclic AMP (cAMP) analogues, including newly synthesized benzimidazole ribofuranosyl 3',5'-monophosphates was used to map the essential molecular interactions between cAMP and the monoclonal antibody 4/2C2 (mab 4/2C2) directed against 2'-O-succinoyl cAMP [Colling, Gilles, Nass, Moka & Jaenicke (1988) Second Messengers Phosphoproteins 12, 123-133]. Its paratope binds the purine base in syn conformation by dipole-dipole interactions and hydrophobic forces and/or stacking interactions. The ribose phosphate moiety is recognized by a combination of charge interactions and H-bonds to the exocyclic and the 5'-oxygen atoms and a hydrophobic interaction at the 2'-position. There is no regioselectivity for the exocyclic oxygen atoms. Compared with the known types of binding, mab 4/2C2 thus shows a new combination of molecular interactions which may be the basis of its strikingly specific recognition and binding of the cyclic adenylates. On this account mab 4/2C2 may become an important tool in studies on cAMP metabolism.