Nine pigs were inoculated oronasally with 10(8.0) TCID50 Aujeszky's disease virus (ADV). Blood was examined daily for the presence of a viremia during 7 days post inoculation. Cell-free and/or mononuclear cell (MC)-associated ADV was demonstrated in 7 out of the 9 pigs. The number of days on which MC-associated ADV was detected per pig ranged from 1 to 4 days. The number of infected cells ranged between 1 and 200 per 10(8) examined MC. Monocytes were found to be the most ADV-susceptible MC in vivo. Interactions of ADV with MC were also examined in vitro. Subpopulations of MC were inoculated with ADV at a multiplicity of infection of 20. Depending on the method of cell separation, 2 or 21% of the monocytes were infected. Lymphocytes were rather refractory to ADV-infection.