Induction of immune response as characterised by expression of cytokine genes in the spleen following immunization of pre- and post-hatch chickens with herpesvirus of turkeys (HVT) vaccine was studied. The pattern of expression of IFN-gamma and IL-10 genes in pre-hatch immunized chickens was different from that observed in post-hatch HVT immunized chickens. This expression pattern of cytokine genes was associated with significantly higher HVT transcripts in pre-hatch immunized chickens than in post-hatch immunized chickens. In conclusion, HVT immunization in chickens, irrespective of the age of immunization, stimulates host response characterised by the expression of cytokine genes, such as IFN-gamma and IL-10 in the spleen. However, the age of immunization appears to influence the temporal pattern of IFN-gamma and IL-10 expression as well as replication of HVT.