A highly efficient, visible wavelength (lambda (MAX) = 700 to 800 nm) fluorescence is reported for four water-soluble monolayer-protected gold clusters (MPCs). Fur 451 nm excitation. the quantum Yield of the luminescence of MPCs with 1.8 nm diameter cores and protected by monolayers of tiopronin thiolate is estimated as 0.003 +/- 0.001. The efficiency and wavelength of the luminescence Gary with the ligands of the monolayer around the gold core. The mechanism for the luminescence is hypothesized to be associated with interband transitions between the filled 5d(10) band and 6(sp)(1) conduction band.