The Borrelia burgdorferi outer surface lipoprotein OspA is a current focus for vaccine development to prevent Lyme disease infection. A soluble, recombinant form of the protein lacking the amino-terminal lipid membrane anchor was cocrystallized with the Fab fragment of an agglutinating mouse monoclonal antibody. The crystals belong to space group P2(1)2(1)2(1), with a = 90.0 Angstrom, b = 91.9 Angstrom, and c = 102.9 Angstrom and they were found to diffract to a maximum resolution of 2.8 Angstrom using synchrotron radiation. (C) 1995 Academic Press, Inc.