The flagellum associated polypeptide p41 is an immunodominant antigen of Borrelia burgdorferi in the early and late stages of Lyme borreliosis. p41 was prepared by affinity chromatography using monoclonal antibody specific for p41. An immunoglobulin class specific ELISA (IgM-, IgG-ELISA) was established with purified p41 as antigen and compared to the conventional ELISA with whole cell ultrasonic antigen. Whereas the sensitivity of IgM- and IgG-ELISA was comparable in both antigen preparations, crossreactivity of sera from syphilitic patients was reduced in the p41 IgG-ELISA. Discrepant results obtained by use of ultrasonic antigen or p41 antigen, were controlled by Western blots. A correlation between the results of p41-ELISA and Western blot was shown.