Our objective was to characterize further the acute-phase response following endotoxin (i.e. lipopolysaccharide; LPS) exposure in the bovine. Nine pure-bred Angus castrated males (i.e. steers; average body weight=299+/-5 kg) were used in a randomized complete block design in environmentally controlled chambers, set at thermoneutral level, to characterize the acute physiological, endocrine, immune, and acute-phase protein responses following an i.v. bolus administration of 2.5 microg of LPS/kg body weight. One day before administration of LPS, all steers were fitted with an indwelling jugular vein catheter for serial blood collection. Blood samples were collected at 30-min intervals from -2 h to 8 h relative to the LPS challenge (time 0), and serum was harvested and stored at -80 degrees C until analyzed for concentrations of cortisol, pro-inflammatory cytokines, and acute-phase proteins. Indicators of thermal status (i.e. rectal temperature, ruminal temperature, respiration rate, sweat rate, and skin temperatures) were measured at 30-min intervals from -1 h to 6 h relative to the challenge. Endotoxin exposure increased (P<0.05) serum concentrations of cortisol, tumor necrosis factor-alpha (TNF-alpha), interleukin 1-beta (IL-1beta), IL-6, interferon-gamma (IFN-gamma), and serum amyloid A. Respiration rate, rectal temperature, and rump skin temperature also were increased (P<0.05) following LPS administration. Endotoxin exposure dramatically decreased ear skin temperature (P=0.002), but tended to increase (P<0.10) ruminal temperature, shoulder skin temperature, and shoulder sweat rate. Serum concentrations of acid soluble protein, alpha-acid glycoprotein, IL-4 and IL-2, and rump sweat rate were not altered (P>0.24) by the challenge. To our knowledge, this report is the most complete characterization of the bovine acute-phase response to a bolus-dose endotoxin challenge conducted under thermoneutral conditions and should provide foundation data for future research.