A potential risk in pig-to-human xenotransplantation is the transmission of PERVs to human recipients. Here we show for the first time the inhibition of PERV expression in primary porcine cells by RNA interference using lentiviral vectors. Cells were transduced with lentiviral vectors coding for short hairpin (sh) RNAs directed against PERV. In all primary porcine cells studied and in the porcine kidney cell line PK-15, expression of PERV-mRNA was significantly reduced as measured by real-time PCR. Most importantly, expression of PERV proteins was almost completely suppressed, as shown by Western blot analysis. Thus, lentiviral shRNA vectors could be used to knockdown PERV expression and create transgenic pigs with a reduced risk of PERV transmission during xenotransplantation.