Arthropod-borne members of the genus Borrelia cause relapsing fever and Lyme disease. We are taking biochemical and molecular biological approaches to the study of pathogenesis of these human disorders. With regard to the antigenic variation in relapsing fever, we have identified and characterized the variable antigens and are establishing the molecular genetic basis for the variation. In our studies of Lyme disease, we identified several antigenic components of the etiologic spirochete, developed monoclonal antibodies to these components, and cloned genes for surface antigenic proteins into E. coli.