Reversible electrochemistry of horse heart cytochrome c (Cc) has been obtained at SnO2 electrodes modified with 12-nm-diameter colloidal Au particles. Previous experiments had demonstrated electrochemical addressability of these particle ensembles; the high current densities and small peak-to-peak separations observed for Cc are indicative of facile electron transfer. In contrast to previously described modified electrodes for Cc, these data were acquired without polishing and without any modification of the Au particle surface. Quasireversible voltammetry was obtained with surfaces comprising monodisperse 36-nm-diameter and polydisperse 6-nm-diameter Au particles, but no voltammetric wave for Cc was seen at surfaces composed of aggregates of 12-nm-diameter or 22-nm-diameter Au particles. These data indicate that nanometer-scale morphology of metals plays a key role in protein electrochemistry, and suggest that isolated, surface-confined colloidal Au particles may be useful building. blocks for macroscopic metal surfaces for biological applications.