A rabbit was immunized with rabbit immunoglobulins of a different allotype. The anti‐allotypic antibodies produced by this rabbit were used to immunize a second rabbit which produced anti‐idiotypic antibodies. To explain the occurrence, among these anti‐idiotypic antibodies, of “internal images” of the original immunizing allotype, a restricted and a more general hypothesis are developed. The first assumes that B‐cells can be triggered when idiotopes on their receptor molecules are recognized by the paratopes of the immunizing antibody. The second denies the existence of a specially constructed combining site on the variable domain of an antibody molecule.