Borrelia burgdorferi sensu stricto strain 297 and B. garinii strains HP1 and 12-92 were serially subcultured for 36-50 passages in vitro for 1 year. All low-passage strains showed abundant expression of outer surface protein C (OspC) in the 22-23-kDa range, but the high-passage strains lost or showed reduced expression of OspC in comparison with the low-passage strains. The low-passage strains efficiently infected outbred ddY mice when inoculated into the hind footpad or peritoneal cavity. In contrast, the incidence of infection with the high-passage strains was low. Isolates from the bladders of mice inoculated with the high-passage strains expressed large amounts of OspC in comparison with those originally inoculated. These results indicate that OspC expression is related to the infectivity of Lyme disease borreliae.