In vitro and in vivo replication of Aujeszky's disease virus (ADV) in swine alveolar macrophages (AM) was studied using two virulent strains and a vaccine strain with deletions in the thymidine kinase and gIII genes. In vitro, AM were highly permissive to virulent ADV infection. Virus progeny titers of virulent strains in the cell phase and in the fluid phase were higher than 10(7.3) TCID50/ml at 84 hr post-inoculation (PI). For vaccine strain infection, AM were less permissive, yielding virus titers of 10(2.3) TCID50/ml at 84 hr PI. To study in vivo replication of ADV in AM, virus isolations were made from AM collected at intervals from pigs inoculated intranasally with both the virulent and vaccine strains. Virus was isolated from AM samples collected from all pigs infected with the virulent strain from days 2 to 22 PI. On the contrary, no virus was detected in AM samples collected from pigs infected with the vaccine strain. The results presented suggested that in vivo virulent ADV replicates for a relatively long period in swine alveolar macrophages.