Within an immunological network consisting of interacting paratopes and idiotopes, three families of molecules can be postulated which bear a peculiarly close connection to any given epitope; the antibody, which carries paratopes which fit the epitope; the anti-idiotype, which carries paratopes fitting the idiotopes of the antibody; and the "homobody", which carries idiotopes to which the paratopes of the antibody fit. The homobody, called by Jerne the "internal image set", is a molecule which resembles the epitope, not like a mould resembles its cast, but like one cast resembles another cast made in the same mould. A distinction between homobody and anti-idiotype poses certain problems which are discussed in the light of a hypothetical experiment using avidin as the antigen.