Two-animal rule: A rule instituted by the US Food and Drug Administration (FDA) for the licensing of agents directed against diseases of low or no incidence. The two-animal rule states that, when agents such as an antibiotic or vaccine are needed for such diseases, these agents may be licensed if they meet two criteria. One criterion is that they show adequate protection against a deliberate challenge of infection in two species of animals, one being a non-rodent species. (This should ideally be a nonhuman primate.) And the other criterion is that the agent is safe in humans.