Synthesis of IFNa by leukocytes of pigs was tested in vitro by incubating diluted total blood in the presence of pseudorabies virus (PrV, Suid Herpesvirus 1)-infected, glutaraldehyde-fixed, PK15 cell monolayers, according to a protocol previously described for transmissible gastroenteritis virus [39] . Confluent PK15 cell monolayers grown in 24 well plates were infected by PrV at a multiplicity of infection of 20, fixed with 0.05% glutaraldehyde 8 h post-infection and washed with D-PBS and RPMI1640 before the addition of blood samples. For each animal, monolayers were incubated with diluted heparinized blood (270 ml of blood diluted 1:5 in DMEM supplemented with antibiotics) for 18 h at 37uC. Plates were then centrifuged at 450 x g for 20 min at 4uC, and supernatants were collected and stored at -20uC. IFNa was assayed in the supernatants using a classical sandwich ELISA test as previously described [40] .