We prepared and crystallized five complexes of the human histocompatibility molecule HLA-A2 with peptides derived from human immunodeficiency virus type 1, human T lymphotropic virus type 1, influenza A virus and hepatitis B virus proteins. Each HLA-A2 complex was refolded in vitro from insoluble proteins produced in bacteria; to crystallize, two of the complexes required seeding with microcrystals of another complex. Maintained at -160 degrees C, single co-crystals of each of the five peptide-HLA-A2 complexes yielded complete X-ray diffraction data sets to a resolution of approximately 2.5 A. After a sufficient number of diffraction peaks were acquired during data collection, the direct analysis of integrated intensities established the point group of the co-crystal, thus allowing an efficient data collection strategy to be designed. The subsequent examination of systematic absences revealed that the five peptide-HLA-A2 co-crystals formed in space groups P1, P2(1), or P2(1)2(1)2(1). Molecular replacement structure solutions yielded unambiguous protein electron density maps, thus confirming the space group determinations. The system of obtaining HLA-A2 co-crystal structures described here is applicable to other crystallographic problems where structures of several related molecules from uncharacterized single crystals are required.