Two primary broiler breeder lines, A and B, were examined for their potential to produce nitric oxide (NO) after stimulating splenocytes from 20-day-old embryos with lipopolysaccharide and interferon-gamma. Significant differences were found between lines A and B. Overall, line A had a higher response than line B, but line A also had a large degree of variation between individual sire families. Selection for high and low responders within line A resulted in the segregation of high- and low-responder sire families. Offspring from sire families selected for high and low NO responses and from a nonselected control group from line A were challenged with RB-1B Marek's disease (MD) virus to determine whether these differences could be used to select for improved resistance to MD. Virus isolation rates at 6 and 10 days postinfection were not significantly different, but unexpectedly, the MD incidence in the high-responder group was significantly higher than in the other two groups.