Polyclonal antibodies, raised against cyclic AMP (cAMP) by the immunization of animals with a 2'-O-succinyl cAMP/bovine albumin conjugate, have been reported to be dependent upon the presence of calcium ion (Ca2+) for antigen binding. They also exhibit a major "bridge" effect whereby 2'-O-succinyl and 2'-O-acetyl derivatives are bound more avidly than the parent nucleotide. Since cAMP and these derivatives bind Ca2+ very weakly, they do not present substantially in the chelated form over the range of Ca2+ concentrations used. Thus direct antigen modification is excluded as an explanation for the observed ion dependence of the reaction. Instead, we propose a mechanism based on reaction coupling. The actual antigens are the Ca2+ chelates of these nucleotides, whose formation in the absence of antibody is rapid but not favored (as indicated by their weak association constants). When antibody is added, the chelates act as transient intermediates whose concentration remains low but which is replenished as they are consumed by antibody. The coupled reaction is driven by the antibody-antigen step which occurs more slowly but with a substantial gain in free energy. The reaction is limited by the availability of Ca2+. It also appears that the rabbit antibody-forming cell responds preferentially to the Ca(2+)-bound form of the 2'-O-succinyl cAMP/bovine albumin conjugate which may appear to be more "foreign" than the unbound form of the hapten containing the ubiquitous nucleotide cAMP.