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The functional properties of humoral factors generated in rats immunized against Borrelia burgdorferi were investigated. After Lewis strain rats were injected intraperitoneally with live B. burgdorferi or in the footpad with dead borreliae incorporated in complete Freund’s adjuvant, they produced high-titered antisera. At a dilution of less than or equal to 1:10, sera from immunized or infected rats but not control sera inhibited growth of B. burgdorferi in vitro. Neutralization of growth of three different strains of B. burgdorferi by posttreatment sera was dose-dependent and was detected equally well by direct microscopic counts or by measuring incorporation of tritiated adenine into newly synthesized nucleic acids. These findings provide direct evidence that infection or immunization with the
Lyme disease spirochete induces the formation of serum factors capable of preventing the growth of B. burgdorferi in vitro.