How an amino acid sequence encodes the information necessary for a protein to adopt a unique tertiary structure remains unresolved. We are addressing this problem by designing "from scratch" protein molecules that will adopt predetermined three-dimensional structures. Based on this strategy, two identical four-stranded beta-sheets were designed to dimerize and form a beta-sandwich protein, called betadoublet. A synthetic gene encoding half the beta-sandwich protein was expressed in Escherichia coli, and the protein was purified to homogeneity. Biophysical characterization of betadoublet in aqueous solution demonstrated that the disulfide formed between the two sheets and that the dimer was a compact unaggregated globular protein, consisting predominantly of beta-sheet and stable to thermal denaturation. It has some backbone amide protons whose exchange is slow enough to be measured by NMR but binds more of the dye 1-anilinonaphthalene-8-sulfonate than a well-folded protein.