We studied the susceptibility of term placental trophoblast cells to in vitro infection with human T cell leukaemia/lymphotropic virus type I in order to provide further insight into the role of syncytiotrophoblast in transplacental passage of the virus. Pure villous trophoblast cultures were exposed to cell-free virus and the extent of infection was analysed by semiquantitative PCR assay to detect integrated proviral DNA. Four different primer pairs targeting the gag, pol, env and pX regions invariably revealed that virus sequences were present in amounts 10²–10³ times less than in the reference cell line MT-2. Virus expression was studied at both the transcriptional and translational levels. Whereas doubly spliced mRNAs coding for the Tax and Rex regulatory proteins could be detected by RT-PCR no virus-specific proteins were found in the cells by immunoperoxidase staining. The present data lend support to the notion that the placental trophoblast may represent a barrier effectively protecting the fetal compartment from exposure to the virus.