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The immunogenicity of a single dose of a whole-cell vaccine of inactivated Borrelia burgdorferi HSF (human spinal fluid isolate) was evaluated. Syrian hamsters were vaccinated subcutaneously and challenged by the intraperitoneal injection of 1,000 50% infectious doses of B. burgdorferi HSF 30 or 90 days postvaccination. Animals were sacrificed 14 days after challenge, and the kidneys and spleens were examined for spirochetes by cultural procedures. At 30 days postvaccination, 86 to 100% protection against infection was achieved in hamsters receiving 50 and 100 micrograms (dry weight) of vaccine. Protection was decreased to 60% with 25 micrograms of vaccine and was absent with 10 micrograms of vaccine. Resistance to infection decreased to 25, 40, and 5% for the 100-, 50-, and 25-micrograms vaccine doses, respectively, at 90 days postvaccination.