The beta-sandwich cupredoxin Plastocyanin (Pc) was found to self-assemble in the presence of Zn2+, a known mediator of protein-protein interfaces. Diffraction-quality crystals of Pc grew from solutions containing zinc acetate as the sole precipitant. Di- and trinuclear zinc sites contribute to the crystal contacts in this structure. A different crystal form, also involving numerous zinc bridging ions, was obtained in the presence of poly(ethylene glycol) 8000. Comparison of the two crystal forms reveals the effect of macromolecular crowding on self-assembly. Solution-state structural characterisation of the Zn2+-mediated Pc oligomers was performed by using a combination of chemical shift perturbation mapping and small-angle X-ray scattering. The data indicate the formation of dimers in solution. The implications for metal-mediated assembly and crystallisation are discussed.