An unprecedented small thioaurite cluster compound (with metallic Au-0 core) has been isolated in high yield by decomposition of polymeric Au(I)SG compounds, where GSH is the ubiquitous tripeptide glutathione, N-gamma-glutamyl-cysteinyl-glycine. The Au:SG clusters appear to share the high stability and robustness of their hydrophobic n-alkyl analogues but are highly water soluble. The most abundant cluster produced by these methods can be easily separated from its homologues by gel electrophoresis. Its total molecular weight is ca. 10.4 kDa, and the mass of its strongly bound inorganic core is 5.6 kDa, suggesting the composition AU(28)(SG)(16) This composition is also consistent with the X-ray diffraction pattern of the crystalline molecular solid. Distinct features in the optical absorption spectroscopy are inherently different from either larger clusters or smaller gold duster compounds. The compound is optically active, as evidenced by circular dichroism in the near-iii, visible, and near-UV regions. The C-13 NMR spectra suggest that the bonding environment of the GS-adsorbate is similar to that of the n-alkyl-adsorbate dusters, and the nonsulfhydryl properties are retained. The cluster is thus envisioned as a large metallic-cluster compound with distinctive optical properties encapsulated by a bioactive peptide monolayer.