Are substances that test positive with the Ames test necessarily carcinogenic in humans?

Not necessarily. Substances that appear to be mutagenic by the Ames test will require subsequent testing on higher-ordered organisms, such as animals. The Ames test assesses mutagenicity on bacterial colonies, which are prokaryotic cells. These lower-model organisms are not a perfect match to eukaryotic mammalian cells. Therefore, a substance classified as a mutagen by the Ames test might not necessarily be harmful to humans and further tests are required for any potential carcinogen identified in the test.