Porcine endogenous retroviruses (PERVs) are of particular concern with xenotransplantations using pig cells, tissues or organs as they are present in the genome of all pig strains and are able to infect human cells in vitro. However, it remains unclear whether PERV particles will be produced in vivo and whether they may infect xenotransplant recipients. Since normal pig peripheral blood mononuclear cells (PBMCs) may be transmitted together with the transplanted organ, the production of PERVs by stimulated PBMCs was studied in vitro. To simulate antigen-induced activation of PBMCs, phytohaemagglutinin (PHA), a T cell mitogen, and the phorbol ester O-tetradecanoylphorbol-13-acetate (TPA), a tumour promoter, were used. Virus release was estimated by measuring reverse transcriptase (RT) activity and by RT-PCR of pelleted viruses. Treatment of pig PBMCs with PHA or TPA induced the release of PERVs. For the first time, a correlation between the extent of proliferation of pig PBMCs and PERV production was shown. In addition, PERV release by non-proliferating cells and differences in virus production between stimuli as well as between different pig strains and individuals of one strain were observed. Subtype analysis revealed the release of the three subtypes PERV-A, PERV-B and PERV-C. In contrast to murine endogenous retroviruses, PERVs were induced by PHA alone. The data suggest that the PBMCs transmitted within a xenotransplant may release PERV. These data also suggest that pig strains producing low amounts of virus could be more suitable for xenotransplantation.