Anti-ribonuclease antibodies (Ab1) of one rabbit were used to prepare in two other rabbits anti-idiotypic antibodies (Ab2) against them. Anti-idiotypic antibodies (Ab3) were prepared against Ab2 of one rabbit in two other rabbits that had the same phenotype as the former three for the allotypic a and b systems. After a rest period of 4 months, the latter two rabbits were immunized against ribonuclease. The antibodies produced (Ab1') crossreacted with the two anti-idiotypic sera (Ab2) against Ab1, showing an idiotypic similarity between anti-ribonuclease Ab1' and anti-ribonuclease Ab1. No such crossreaction was observed between the anti-idiotypic sera (Ab2) against Ab1 and any of the 12 unrelated antisera against ribonuclease that were tested. These results suggest (i) that rabbits, at least with the same allotypes, possess a closely related idiotypic repertoire; and (ii) that the immune system is a network of variable (V)-domains.