Mice immunized with recombinant outer surface protein A (OspA) in Freund's adjuvant or with intraperitoneal injections of live Escherichia coli expressing OspA have been shown to be protected from infection with Borrelia burgdorferi. To investigate the efficacy of oral vaccination, C3H/He mice were inoculated with 10(8) live E. coli expressing recombinant OspA by gavage and boosted in a similar manner on days 10, 20, 30, and 40. The animals developed serum IgG antibodies to OspA by immunoblot and were protected from infection when challenged with 10(4) B. burgdorferi intradermally 14 days after the last boost. Control mice did not develop antibodies to OspA and were not protected against challenge infection. These results suggest that an oral preparation of recombinant OspA could potentially be used for vaccination.