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This paper describes for the first time the differential immune response to virulent Newcastle disease virus (NDV) in birds differing in cell-mediated immunity, as measured by response to phytohaemagglutinin-P. To explore potential host-pathogen interactions, peripheral blood mononuclear cells (PBMC) were collected from 40 extreme responder birds (20 birds each from high and low cell-mediated immunity lines). PBMC cultures were stimulated by virulent NDV and temporal expression profiles of interferon-gamma (IFN-gamma), and inducible nitric oxide synthase (iNOS) mRNA was evaluated by semiquantitative reverse transcription polymerase chain reaction (PCR). To further explore the correlation of iNOS mRNA expression and nitric oxide (NO) production, we assayed the culture supernatants for NO. NO production, as well as iNOS and IFN-gamma mRNA expression, was significantly (P < 0.05) higher in the line with higher cell-mediated immunity. In our study, a significant (P < 0.05) difference was observed between the lines for IFN-gamma promoter polymorphism for the TspEI site. The high cell-mediated immunity line mostly revealed the genotype (GG) with a 168-bp fragment. On the other hand, this genotype was not predominant in the low cell-mediated immunity line. Later, quantitative real-time PCR demonstrated higher (P < 0.01) IFN-gamma mRNA transcription in the genotype GG in response to NDV. This difference in promoter region may be responsible for differential IFN-gamma mRNA transcription in chicken lines. Furthermore, birds of high cell-mediated immunity line showed better adaptive immunity to booster NDV vaccination as revealed by an enhanced antibody titre. Thus, this study provides baseline data on the effect of phytohaemagglutinin-P response-based selection on immune responses to virulent NDV and the data could be of immense importance to poultry geneticist and immunologist attempting to breed poultry for disease resistance.