A number of monoclonal antibodies were derived from the spleen cells of dinitropheryl (DNP)immunized mice. Both T-dependent and T-independent carriers were used, and the intensity and length of immunization were varied. It was found that some of the antibodies had only idiotypic (Ab1) reactivity, while others had both idiotypic (Ab1) and anti-idiotypic (Ab2) reactivity. Among the latter antibodies some molecules reacted specifically with DNP and with the combining site of anti-DNP antibodies (epibodies), while others bound DNP and anti-DNP Abs as well as a variety of unrelated antigens (polyreactive antibodies). The proportion of the three types of antibodies (antigen-specific, epibodies and polyreactive antibodies) varied with the nature of the carrier, the intensity of the immunization, and the length of the immunization process. Further characterization of the epibodies, which were predominant in the secondary response to DNPkeyhole limpet haemocyanin (KLH), showed that both Ab1 and Ab2 reactivities were inhibited by both soluble ligands (DNP and anti-DNP), indicating that the specific combining site of the monoclonal antibodies (mAbs) (and/or of the rabbit anti-DNP antibody in the case of Ab2) was involved in both activities. Both Ab1 and Ab2 reactivities were removed by absorption of the mAbs with either immobilized DNP or immobilized rabbit anti-DNP. The mAbs were capable of binding themselves as well as to other mAbs with the same characteristics. The affinity constants of several mAbs for both the DNP and anti-DNP ligands were determined.