The etiological agent of Lyme disease (LD) is a tick-borne, hitherto unknown spirochete of the genus Borrelia. We are studying pathogenesis of LD, and to obtain useful reagents for this study as well as to understand the biology of the etiological agent, we have initiated a molecular approach. To date we have prepared a gene bank of LD spirochete DNA in Escherichia coli and have isolated three recombinant clones expressing LD spirochete antigens. One of the clones produces both of the major, outer-membrane associated proteins of LD spirochete B31. These and other clones will be examined in depth and will be used to assess the feasibility of immunoprophylaxis of LD.