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Mice vaccinated with outer surface protein A (OspA) from Borrelia burgdorferi strain N40 are protected from challenge with an intradermal syringe inoculum of B. burgdorferi strains N40, B31, and CD16. Vaccination experiments were done to determine if protection extended to strains 297 and 25015. We now show that OspA-N40 immunized mice are protected against challenge with strain 297, isolated from the cerebrospinal fluid of a patient with neuroborreliosis, but not against challenge with strain 25015, isolated from a tick in Millbrook, NY. The OspA gene from strain 25015 was therefore cloned and sequenced. The deduced OspA-25015 protein sequence differs from OspA-N40 at 40 of 273 amino acids. Furthermore, mice vaccinated with rOspA-25015 are protected from challenge with strain 25015 but not against strain N40. The results extend the usefulness of OspA as a vaccine candidate, but indicate that OspA can vary among strains of B. burgdorferi and that vaccination of mice with OspA-N40 does not protect against intradermal challenge with an inoculum of 10(4) strain 25015 spirochetes.