Release tags are novel compounds made up of three molecular groups: "signal," "release," and "reactivity." The signal group is the detected part of the tag, the release group provides a site for specific covalent cleavage, and the reactivity group attaches the tag to a substance of interest. The general purpose of release tags is to function as molecular labels, analogous to radioisotopes. Release tags also extend the applicability of gas-phase electrophores as signal groups to the analysis of nonvolatile substances. These electrophores are measured with high sensitivity and specificity by gas chromatography with electron-capture detection (GC-ECD). Because extensive multiplicity is potentially available with GC-ECD electrophores, many analogous but distinctive release tags can be used simultaneously as labels in a given assay. In this paper, we use N-pentafluorobenzoylmethionylglycine-N-hydroxysuccinimide ester, our first GC-ECD release tag, to determine thyroxin in serum by a method called "labeling analysis," which involves the principle of isotope-derivative analysis.