The dynamic interplay between preparation, handling, containment, and analysis of ultrapure chemical standards and reagents for the clinical chemistry laboratory is described. Inorganic as well as organic standards are reviewed. The contamination problems associated with water, acids, and solvents in trace-element analysis are defined and resolved. In addition, less well-known sources of error in trace analysis are reviewed, as are the common contaminants in the clinical laboratory. It does little good to use a 99.99% primary standard with water of unknown quality in a heavily contaminated laboratory. Fortunately, recognition, definition, and solution of this problem is possible and should introduce new levels of precision and accuracy in the clinical chemistry laboratory.