To determine if the immunoglobulins (Igs) capable of eliciting the formation of isologous anti-idiotypic antibodies are rare exceptions, BABL/c mice were immunized with five myeloma proteins of BALB/c origin. Anti-idiotypes were produced against all but one. The idiotype of the exception (T15) is remarkably abundant in BALB/c mice, whose unresponsiveness is probably due to tolerance. Nevertheless, prolonged immunization with T15 finally induced the formation of isologous antibodies that seemed largely to be specific for IgA proteins, especially those with k-light-chains; the reactions of a few of these isologous antisera with T15 were slightly inhibited by phosphorylcholine, suggesting that some anti-idiotypes were probably formed even to this unusually prevalent idiotype. It is likelythat under appropriate conditions almost any myeloma protein can elicit isologous anti-idiotypes.