Monoclonal antibodies to cyclosporine A (Cs), a potent immunosuppressant, were generated in BALB/c mice using a novel antigen prepared by linking Cs to a protein carrier via a photoactive cross-linking reagent, 4-benzoylbenzoic acid (BBa). Twenty-two monoclonal anti-Cs antibodies were generated, using Cs-BBa-bovine serum albumin (Cs-BBa-BSA) as the immunogen. They were characterized with respect to affinity by Scatchard analysis of a radioimmunoassay (RIA), and with respect to specificity by an ELISA in which a series of singly substituted Cs derivatives were examined as inhibitors. McAb affinities ranged from 5 x 10(-8) M to 2 x 10(-10) M. Based on ELISA inhibition data with Cs analogs, and on the binding to two Cs-BSA conjugates in which opposite sides of the Cs molecule are exposed, the antibodies fell into five epitope recognition groups. Binding to Cs was also studied by ELISA in competition with cyclophilin (CyP), a Cs-binding protein whose epitope specificity has been well characterized. Competition by CyP was found to correlate with antibody specificity, not with affinity, i.e. CyP competed best with antibodies having specificities most similar to that of CyP. Epitope mapping can, therefore, be accomplished in a system in which two different species of binding proteins compete for the same antigen. This type of characterization may be useful in identifying antibodies whose combining sites mimic those of a receptor.