The current vaccine against tuberculosis, Mycobacterium bovis strain bacille Calmette-Guerin (BCG), offers potential advantages as a live, innately immunogenic vaccine vehicle for the expression and delivery of protective recombinant antigens (Stover, C.K., V.F. de la Cruz, T.R. Fuerst, J.E. Burlein, L.A. Benson, L.T. Bennett, G.P. Bansal, J.F. Young, M.H. Lee, G.F. Hatfull et al. 1991. Nature [Lond]. 351:456; Jacobs, W.R., Jr., S.B. Snapper, L. Lugosi and B.R. Bloom. 1990. Curr. Top. Microbiol. Immunol. 155:153; Jacobs, W.R., M. Tuckman, and B.R. Bloom. 1987. Nature [Lond.]. 327:532); but as an attenuated intracellular bacterium residing in macrophages, BCG would seem to be best suited for eliciting cellular responses and not humoral responses. Since bacterial lipoproteins are often among the most immunogenic of bacterial antigens, we tested whether BCG expression of a target antigen as a membrane-associated lipoprotein could enhance the potential for a recombinant BCG vaccine to elicit high-titered protective antibody responses to target antigens. Immunization of mice with recombinant BCG vaccines expressing the outer surface protein A (OspA) antigen of Borrelia burgdorferi as a membrane-associated lipoprotein resulted in protective antibody responses that were 100-1,000-fold higher than responses elicited by immunization with recombinant BCG expressing OspA cytoplasmically or as a secreted fusion protein. Furthermore, these improved antibody responses were observed in heterogeneous mouse strains that vary in their immune responsiveness to OspA and sensitivity to BCG growth. Thus, expression of protective antigens as chimeric membrane-associated lipoproteins on recombinant BCG may result in the generation of new candidate vaccines against Lyme borreliosis and other human or veterinary diseases where humoral immunity is the protective response.