Cyclic voltammetry of Pseudomonas aeruginosa azurin on polycrystalline gold is reversible (E-0 = 360 m V vs she : 50 mM ammonium acetate) but the voltammetric signals decay with time constants of about 3 x 10(-3) s(-1). No signal is observed for monocrystalline Au(lll). Cys3Ser azurin is electrochemically inactive on either type of gold electrode but shows a reversible although decaying peak (362 mV, 50 mM ammonium acetate; decay time constant approximate to 2 x 10(-3) s(-1)) on edge-plane pyrolytic graphite. Ex situ and ill situ atomic force microscopy (AFM) of the azurins on Au(1 1 1) show initially arrays of protein structures of lateral 100-200 Angstrom and vertical approximate to 50 Angstrom extension. These could be individual molecular images convoluted with the tip curvature. As scanning proceeds the structures in the ex situ mode collect into large two-dimensional assemblies while the adsorbed protein in the in situ mode is largely swept into the solution, recovering the free Au(lll) surface. The cyclic voltammetry and AFM data are consistent with time dependent adsorption of the azurins on gold ria the disulphide bridge (wild-type) or free thiol group (Cys3Ser mutant).