A B10.A (5R) responder mouse to the random linear terpolymer, poly--(Glu, Lys, Phe), GLphi, can produce immunoglobulins which bind poly-L (Glu, Lys), GL, that share idiotypic determinants with GL-binding antibodies produced by other members of the same strain. Expression of these common idiotypic determinants, termed BGL, is independent of the H-2 halotype and closely linked to the Ig-lb heavy chain allotype. Moreover, nonresponder mice with the Ig-lb heavy chain allotype, when immunized with GLphi that has been chemically coupled to an immunogenic carrier, chicken IgG, can produce GL-binding antibodies that share BGL idiotypic specificities with anti-GLphi antibodies produced by responder animals. Also, the responses to other GL-containing polymers, such as poly-L (Glu, Lys, Ala) and poly-L (Glu, Lys, Pro), which are under the control of distinct Ir genes, can stimulate the production of GL-binding antibodies that share common BGL idiotypic determinants with antibodies induced with GLphi. These findings are discussed with respect to their implications concerning the mechanism(s) of Ir gene control.