We compared inflammatory responses to lipopolysaccharide (LPS) injection in laying type (Brown Nick) to broiler type (Avian x Avian) chicks. Rectal temperature was measured at 0, 1, 2, 4, 6, 12, and 24h after LPS injection (0, 0.1, 0.3, 0.6, 1, 2.5, or 5mg/kg bw). In layers, rectal temperature increased from 41.31+/-0.19 degrees C to a maximum 42.27+/-0.41 degrees C at 4h after 1mg/kg LPS. Relative to layers, the febrile response in broilers was considerably lower, delayed in onset, and required higher levels of LPS (5mg/kg). Proliferation of spleen cells from un-injected chicks in response to LPS, PHA, and Con A was evaluated in vitro. IFNgamma, TGFbeta(2), MGF and IL-1beta relative to beta-actin mRNA expression were analyzed in spleen cells stimulated with LPS. Splenocytes from layers had a higher proliferative response to LPS (P=0.045), but lower proliferative response to PHA (P=0.004) and Con A (P=0.004) than broilers. Expression of mRNA for MGF, IL-1beta and IFNgamma was lower in broilers than in layers (P<0.001). Reduced production of the pro-inflammatory cytokines in broilers could have resulted from the observed increased production of the immunosuppressive cytokine TGFbeta(2.) These differences in cytokine expression may explain the blunted febrile response in broilers compared to layers. Because the acute phase response of inflammation causes decreased food intake, the blunted inflammatory response of broilers may permit faster growth.