The capabilities of electrospray ionization mass spectrometry are demonstrated for monitoring the flux of metal ions out of and into the metalloprotein rabbit liver metallothionein and, in one example, chlorambucil-alkylated metallothionein. Metal ion transfers may be followed as the reactions proceed in situ to provide kinetic information. More uniquely to this technique, metal ion stoichiometries may be determined for reaction intermediates and products. Partners used in these studies include EDTA, carbonic anhydrase, a zinc-bound hexamer of insulin, and the core domain of bacteriophage T4 gene 32 protein, a binding protein for single-stranded DNA.Keywords: acquired drug resistance; DNA binding protein; electrospray mass spectrometry; insulin; metal ion flux; metallothionein; stress response Metallothioneins (MT) comprise a family of small metal binding proteins that occurs widely throughout the animal kingdom. Plants and bacteria also produce types of metallothioneins. Vertebrate MTs share essentially the same structure (Kagi, 1993), with 20 cysteines coordinating seven divalent metal cations (Schultze et al., 1988; Robbins et al., 1991). TLVO domains are joined by a linker region, the N-terminal or P-domain binding three metal cations and the C-terminal or cy-domain binding four metal cations. Metallothioneins in invertebrate species also contain multiple cysteine side chains that bind metal cations.Although it would seem that a protein as widespread as MT must play a biological role that is essential for the survival of organisms (Vallee, 1995), the function of MT has been a subject for debate since it was discovered 40 years ago (Margoshes & Vallee, 1957). One function often cited for MT is the detoxification of heavy metals, cadmium in particular. However, the incidence of toxic concentrations of heavy metals in the environment is sporadic; thus, the need to detoxify heavy metals does not seem to account for the ability of virtually all living cells to synthesize metallothioneins transcriptionally or enzymatically (Steffens, 1990;