Turnip yellow mosaic virus is an excellent model for eukaryotic positive-stranded RNA virus replication. Correct processing of the replication polyprotein is dependent on the virally encoded cysteine proteinase (PRO) domain. Crystalline needles obtained from highly pure preparations of the recombinant 17.6 kDa PRO did not diffract. In contrast, small hexagonal prisms that were obtained together with the needles under the same conditions but from a poorly purified preparation diffracted to 2 Å resolution and allowed structure determination by MIRAS. It turned out that the hexagonal crystals contained stoichiometric amounts of PRO and Escherichia coli 30S ribosomal S15, a 10.1 kDa protein commonly co-purified by immobilized metal-affinity chromatography. The solvent content is nearly 70%, with S15 bridging parallel infinite helices of PRO across large solvent channels. With hindsight, this spurious interaction not only yielded diffraction-quality crystals but would also have allowed structure determination by molecular replacement using S15 as a search model and subsequent automatic rebuilding of the asymmetric unit.