AbstractBy a uniformly applied protocol, adult hydra are exposed to a test substance over a broad range of concentrations, and the minimal toxic concentration is determined to within one‐tenth log. In a second experiment, dissociated hydra cells are manipulated into a configuration wherein, if undisturbed, they will achieve the developmental events characteristic of any embryo and undergo total whole‐body regeneration. During its 4‐day ontogenesis this artificial “embryo” is exposed to the test substance by the same protocol as the adult and the minimal developmentally toxic concentration is determined to within one‐tenth log. The ratio of the adult (A) to the developmentally (D) toxic concentration is calculated. A small A/D ratio indicates that the substance disrupts development only at or near the concentration also toxic to the adult (a developmentally nonhazardous substance or coeffective teratogen). A large A/D ratio indicates that a substance disrupts developmental events at a small fraction of the exposure toxic to adults (a developmental hazard). The system is directly predictable of a putative teratogen's hazard potential (A/D ratio) in standard laboratory animals and man. It provides an objective and reliable means to prioritize otherwise untested substances according to the need for further study of their development