The bursa of Fabricius serves as an important tissue in the process of Marek's disease virus (MDV) pathogenesis, since B cells of the bursa harbor the cytolytic phase of MDV replication cycle. In the present study, host responses associated with MDV infection in the bursa of Fabricius of chickens were investigated. The expression of MDV phosphoprotein (pp)38 antigen, MDV glycoprotein (gB) and MDV viral interleukin (vIL)-8 transcripts was at the highest at 4 days post-infection (d.p.i.) and then showed a declining trend. On the contrary, the expression of meq (MDV EcoRI Q) gene as well as the viral genome load increased gradually until day 14 post-infection. The changes in viral parameters were associated with significantly higher infiltration of macrophages and T cell subsets, particularly CD4+ T cells into the bursa of Fabricius. Of the genes examined, the expression of interferon (IFN)-alpha, IFN-gamma genes and inducible nitric oxide synthase (iNOS) was significantly up-regulated in response to MDV infection in the bursa of Fabricius. The results suggest a role for these cells and cytokines in MDV-induced responses in the bursa of Fabricius.