Demystifying 21 CFR Part 556--tolerances for residues of new animal drugs in food.

21 Code of Federal Regulations Part 556 (Tolerances for Residues of New Animal Drugs in Foods) is one of the Center for Veterinary Medicine's most significant set of regulations. However, in many respects, it is outdated. Subpart A (General Provisions) defines tolerance designations that are obsolete, while Subpart B (Specific Tolerances for Residues of New Animal Drugs) is inconsistent in terminology and often confusing. The purpose of this paper is to define the older terms and update the reader as to current concepts that apply to tolerance-setting for new animal drugs. A list of useful definitions appears at the end of the article.