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Lyme disease is a vector-borne illness caused by spirochetes belonging to the Borrelia burgdorferi species group. These bacteria employ several mechanisms to survive within the vertebrate host, including evasion of the complement system. In this study, we examine the protection against human serum killing by the binding of host complement regulators C4b-binding protein (C4BP) and factor H (FH) to the bacterial surface of B. burgdorferi. Via serum depletion of isolated complement regulators, we found that the absence of C4BP did not alter the survival of B. burgdorferi strain B31; however, the removal of FH increased the sensitivity of this strain to human serum as previously described. The B. garinii seabird-isolated strain Far04, on the other hand, did not bind any complement regulators of human origin and was serum-sensitive, indicating its special host species specificity.