Serodiagnosis of Lyme borreliosis and syphilis is complicated by antibodies crossreacting with common antigens of the family of Spirochetaceae. Sera from patients with syphilis often contain cross-reacting antibodies againstpartial antigens of Borrelia burgdorferi with molecular weights of80-, 66-, 41-, 39-and 12 kDa. N o cross-reactivity with Borrelia burgdorferi-specific antigen bands is observed. Sera of Lyme disease patients recognize treponemal antigens with 80-, 66-, 60-, 41-and 39 kDa molecular weight, however do not react with antigen bands characteristic ofTreponema pallidum. Serological cross-reactions between Borrelia burgdorferi and Treponema pallidum were tested using the indirect IFA (Inimuno-FluorescentAssay) andSDS-PA GE (Sodium Dodecyl Sulphate Poly-Acrylamide Gel Electrophoresis) with subsequent immunoblotting (Westernblot). After evaluation by various serological tests such äs TPHA (Treponema pallidum Haemagglutination Assay), the FTA-Abs Test (Fluorescence 12