The human placenta serves as a barrier to the transmission of some viruses, but allows others to reach the fetal circulation. The resistance and permissiveness of the placenta to viral transmission appears to be determined in large part by the placental trophoblast. In order to define the mechanisms by which human trophoblast cells influence vertical transmission of viruses, we have studied the interaction of replication-deficient recombinant viral vectors with transformed human choriocarcinoma (BeWo) cells and primary trophoblast cell cultures. Recombinant adenovirus vectors efficiently transduce BeWo cells and primary trophoblast cells. However, as BeWo cells differentiate after treatment with cAMP analogs, they become relatively resistant to adenovirus-mediated transduction due to diminished uptake of the virus particles. This differentiation-dependent loss of adenovirus transduction may be related to the down-regulation of the coxsackie adenovirus receptor, which we have detected in undifferentiated trophoblast cells. Recombinant herpes simplex virus vectors also transduce undifferentiated BeWo cells and isolated cytotrophoblast cells, but transduction by herpes simplex virus vectors declines with cAMP-induced trophoblast differentiation, apparently due to reduced viral uptake. In contrast, cAMP treatment augments trophoblast transduction by recombinant adeno-associated virus, a member of the parvovirus family. This augmentation appears to be due to increased virus uptake by a yet to be determined mechanism. The understanding of the molecular basis of these interactions between recombinant viral vectors and trophoblast cells may yield strategies to prevent vertical transmission of viruses as well as to create opportunities to modify trophoblast function through gene transfer using viral vectors.