Analysis by SDS-PAGE of axial filaments of cultivable treponemes showed 3 major bands: a 33/34 kD doublet and a 37 kD polypeptide. In sera from patients with various stages of syphilis, from biological false positive reactors, and from negative controls the most consistent reaction was detected against the doublet, but only sera from patients with primary or secondary syphilis reacted with the 37 kD axial filament polypeptides. About 6 weeks after antibiotic treatment sera had lost their reactivity with 37 kD antigens in agreement with decreasing antibodies in VDRL-test. Because of their recognition early in infection 37 kD-proteins are postulated to use in serodiagnostic assays, but our data emphasize also the need for use further antigens in tests for diagnosis in later stages of syphilis.