A steady quasi-reversible electrochemical response due to parsley plastocyanin is reported at an edge-plane graphite electrode without the requirement for so-called adjunct-promoters. This observation is unusual in that studies involving plastocyanins from other sources require the addition of polyvalent cations to the solution in order to promote the heterogeneous electron transfer. Here cationic adjunct-promoters are required only under conditions of low ionic strength, where sigmoidal responses are obtained, characteristic of the presence of a low number of electroactive sites on the electrode surface such that conditions of radial, as opposed to linear, diffusion apply.