Oligomers of the chaperonin TF55 from Sulfolobus solfataricus have been successfully crystallized in two dimensions via their interaction with a phospholipid monolayer at the air/liquid interface. Oligomer orientation was dependent upon the lipid headgroup used. A neutral lipid monolayer gave rise to small paracrystalline areas of TF55 side views, whereas a negatively charged lipid monolayer resulted in large coherent crystalline areas of the chaperonin in an end-on orientation. These 2D crystals had p312 symmetry (a = b = 162 Angstrom, gamma = 60 degrees). Two-dimensional projection structures of the end-on arrays were produced by electron microscopy and image processing techniques. Under the conditions used to grow the crystals, the protein formed complexes of two stacked nine-subunit rings with threefold symmetry.