Anti-idiotypic antibodies (Ab2) were raised in allotype-matched rabbits against anti-carbohydrate or anti-tobacco mosaic virus antibodies (Ab1). Several Ab2 were purified and injected into a third series of rabbits III which synthesized antiantiidiotypic antibodies (Ab3). Antigen was then given for the first time in those rabbits who had synthesized Ab3. The specific antibody synthesized in rabbits III was called Ab1'. Anti-idiotypic antibodies were raised against purified Ab3 antibodies (Ab4). In most cases, Ab1' antibodies are sharing idiotypic specificities with Ab1. Ab3 did not react with antigen but shared idiotopes with Ab1 and Ab1' because Ab4 antibodies, which are anti-idiotypes to Ab3 do recognize specifically Ab1 and Ab1' antibodies belonging to the same chain of immunization. It seems therefore that Ab3 looks idiotypically like Ab1 and Ab4 displays the same behaviour as Ab2. A general view of the functioning of the immune system is presented