Borreliacidal antibodies specific for outer surface protein C (OspC) are induced shortly after infection with Borrelia burgdorferi. In this study, we identified the region of OspC recognized by immunoglobulin M (IgM) and IgG borreliacidal antibodies. Sera from patients with early Lyme disease were screened for borreliacidal activity specific for B. burgdorferi 50772 and OspC antibodies. Seven sera that contained similarly high titers of each response were then chosen randomly and adsorbed with OspC or a truncated OspC (OspC-Dra) containing the 50 amino acids nearest the carboxy terminus. Adsorption with OspC or OspC-Dra completely eliminated the borreliacidal activity in six (86%) of seven sera and significantly decreased the activity in the remaining serum (titer of 10,240 to 1,280). Moreover, OspC antibodies were no longer detected by OspC enzyme-linked immunosorbent assay or in a Western blot that contained native OspC. The findings confirmed that sera from patients with early Lyme disease contain high concentrations of IgM or IgG borreliacidal antibodies that bind a conserved region of OspC.