Syngeneic monoclonal anti-idiotypic antibodies were raised against idiotopes on neutralizing monoclonal antibodies with specificity for feline leukemia virus and canine parvovirus. The anti-idiotypic antibodies were shown to recognize paratope-related private idiotopes. Mice were injected with the monoclonal anti-idiotypic antibodies and the sera of these mice were screened for antiviral reactivities. Antibodies to both feline leukemia and canine parvovirus could be induced as determined by ELISA. These results suggest that anti-idiotypic antibodies which detect private idiotopes and thus do not represent internal images of viral antigens may be considered as candidates for the induction of antiviral immunity.