Sera obtained from 14 Lyme borreliosis patients at early stages (stages I and II) of the disease were examined for their borreliacidal properties against Borrelia afzelii isolate FEM1 by use of a growth inhibition assay. Five of 14 immune sera exhibited borreliacidal activity against isolate FEM1. Heat-inactivated immune sera failed to kill the spirochaetes. Immunoblotting experiments with outer-membrane preparations showed that OspC and 11 additional proteins of 14.0, 16.0, 17.7, 19.3, 21.7, 27.5, 32.7, 40.7, 48.9, 51.3 and 53.6 kDa were recognised by borreliacidal immune sera. To analyse the borreliacidal properties of anti-OspC antibodies, two sera (EM4 and EM5), which beside antibodies against a 51.3-kDa protein contained exclusively anti-OspC antibodies, were further investigated by comparative analysis with a FEM1 wild-type and a FEM1 variant lacking OspC in a growth inhibition assay. Only FEM1 wild-type and not variant FEM1OspC(-) was killed by immune sera EM4 and EM5. Complement-dependent killing of FEM1 wild-type was mediated by formation of the terminal complement complex that was found to be attached directly to the outer membrane as confirmed by immuno-electron microscopy. No complement deposition was observed on the surface of variant FEM1OspC(-) after incubation with immune sera EM4 and EM5, thereby suggesting that only anti-OspC antibodies in these two immune sera were responsible for borreliacidal activity. These results provide direct evidence that anti-OspC antibodies, once developed during the immune response, are of critical importance for efficient killing of borreliae in the early phase of infection.