The minimum energy conformations of some isolated and multi-molecular structures of poly-(gly-pro-gly) have been calculated and are presented. The poly-L-proline II helix is not among the four most stable isolated strand conformations, which are numbered here RM 1-4. It is shown that only one of these structures (RM 4) can pack efficiently into a stable crystal form. However, a sacrifice in intramolecular stabilization energy can be recovered in intermolecular interaction for the polyproline II-type conformation. A consideration of these factors leads to at least three possible crystal structures of comparable lattice energy. Each of the structures has an orthogonal unit cell and is sheetlike, but is distinguishable from the others either by chain conformation or packing arrangement. Two of the three structures are compatible with the X-ray evidence for poly-(gly-pro-gly) crystals precipitated from formic acid.