Reconstituted crystalline purple membrane has been prepared starting from denatured bacteriorhodopsin (BR) fragments, native lipids and retinal. The two chymotryptic fragments are thought to contain respectively five and two transmembrane alpha-helices in native BR. The new reconstitution procedure, a modification of that of Huang et al. (1986, J. Biol. Chem., 256, 3802), relies on dodecylsulfate precipitation by potassium ions and yields samples with a high protein-to-lipid ratio (approximately 1:1 w/w). X-ray and neutron diffraction measurements show that in the reconstituted samples BR molecules are arranged in a P3 two-dimensional lattice with the same unit cell dimensions as the native purple membrane lattice. Analysis of reflection intensities indicates that the reconstituted molecules have regained the structure of native BR to 7 A resolution.