Sera from patients with Lyme disease (LD) originating in the United States and Europe were examined by western blot analysis for antibodies to LD spirochete components. Whereas some components reacted with antibodies from the majority of patients, other components, notably an abundant cellular protein with an apparent molecular weight of 34,000 were less commonly bound. This latter proteinaceous component was in a region of the polyacrylamide gel electrophoresis profile which demonstrated variability between different LD spirochete isolates. Thus, there may be more than one serotype of LD spirochete, and the 34,000-range proteins may be a basis for such a distinction.