The study examined the variation in the response to the endotoxin lipopolysaccharide (LPS) in commercial Dekalb chicks or those that were bred for high (HGPS) or low (LGPS) group productivity and survivability (in response to cannibalism and flightiness). Six-week-old chicks housed in colony cages were randomly assigned to either the experimental or the control treatments. The experimental group received an intravenous injection of Escherichia coli LPS (5 mg/kg of body weight), while the control were injected with distilled saline. Clinical symptoms of infection were monitored on 6, 12, 24, 48 and 72 hours after the injection. All the chicks that were injected with LPS exhibited clinical signs of illness. However, the extent of the reaction was genotypically and phenotypically specific. The HGPS chicks underwent brief, but severe behavioural and physical changes, when compared to the Dekalb and LGPS chicks. The body weight gain, organ development and core temperature were most significant in LGPS, lowest in HGPS, while the Dekalb were between the two extremes. The varying responses to the LPS challenges of chicks that underwent different genetic selection indicate that each line adapts differently to stress and their resistance to infection and inflammation. Therefore, when studying the impact that environmental-genetic interactions have on the behaviour and physiology and the subsequent response to stress and disease, the genetic lineage of the chicken needs to be taken into account.