Western blot analysis of the fetal IgM response to Treponema pallidum antigens was examined among 39 pairs of maternal/infant sera; this included 12 mothers and infants with active syphilis (group I), 9 mothers with active syphilis and their infants with uncertain infection (group II), and 18 mothers treated for syphilis before delivery and their asymptomatic infants (group III). A fetal IgM response to T. pallidum antigens with apparent molecular masses of 72, 47, 45, 42, 37, 17, and 15 kDa was observed among sera of infants with congenital syphilis. Fractionation of sera into IgM and IgG components by high performance liquid chromatography confirmed that fetal IgM antibodies in every case were directed specifically against a 47-kDa antigen. Two asymptomatic infants from group II also showed serum IgM reactivities with the 47-kDa antigen, thereby appearing to confirm in utero infection. The combined data suggest that fetal serum IgM reactivity with the 47-kDa antigen of T. pallidum can be used as an important molecular marker for the diagnosis of congenital syphilis.