IgG and IgM antibody specificities for antigens of Treponema pallidum Nichols strain were determined by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the western blot technique in sera from patients with untreated syphilis, normal persons, persons with biologic false-positive tests for syphilis, and sexual contacts of persons with infectious syphilis. IgG reactivities of sera from individuals with primary syphilis varied considerably but consistently exhibited strong reactivity to a 48-kilodalton band. Sera from patients with secondary and early latent syphilis uniformly demonstrated reactivity to 22 separate polypeptide antigens; decreased reactivity was seen in late latent syphilis. Normal and biologic false-positive sera showed weak IgG reactivity against none to 12 polypeptides. Sera from asymptomatic contacts of persons with infectious syphilis showed reactivity to a varying number of treponemal antigens, including some reactions not seen with normal sera. IgM reactivity was most prominent in secondary syphilis but was demonstrable at all stages of disease. © 1985 The University of Chicago. All Rights Reserved.