Infectious bursal disease virus (IBDV) causes highly contagious, immunosuppressive disease that leads to high mortality in young chickens. The purpose of this study was to look for the genetic regulation of the immune acute immune response to IBDV in our selected lines. Chicks of a F2 generation of two lines divergently selected for early high (HH) or low (LL) antibody (Ab) response to Escherichia coLi vaccination were challenged with virulent IBDV. Viral load in infected bursae was used to determine resistant (R) and susceptible (S) birds. By using a 13K chicken cDNA microarray, and pooled spleen mRNA of R, S and non-challenged, control (C) chicks, several genes were identified with differential expression associated with host resistance to IBDV. These genes were also subjected to RT-PCR on individual samples to verify the results obtained from microarrays. The major finding was the co-upregulation of seven genes--ETS2, H963, RGS1, ABIN-2, CREM/ICER, DUSP1 and CXCR4- in several R, but not S or C individuals, and characterized by a high correlation of expression levels. Resistance also generally coincided with reduced transcript levels of acute-phase serum amyloid A (A-SAA) and increased levels of IL-8. Based on reported functions of these genes, these findings suggest that resistance was mediated by the activation of specific cellular mechanisms, indicated by increased activity of splenic macrophages and T-lymphocytes 3 days post-challenge.