High resolution nuclear magnetic resonance spectroscopy has been used to examine the interaction of plastocyanins from French bean (Phaseolus vulgaris) and cucumber (Cucumis sativus) with three complexes--potassium hexacyano-chromate(III), hexamminechromium(III) nitrate and tris(1,10-phenanthroline)-chromium(III) perchlorate--which are analogues of inorganic electron transfer reagents. The results indicate a high degree of specificity in the binding of these complexes and two binding sites on the protein are identified. One binding site is situated close to the copper atom and is clearly suited to outer sphere electron transfer through one of the histidine ligands. The other binding site is more distant from the copper atom and this mechanism cannot be operative. Electron transfer via hydrophobic channels or electron tunneling are possible mechanisms of electron transfer.