Purified lipopolysaccharide (LPS) infusion in cattle induces clinical and metabolic responses similar to gram-negative bacterial infection. Effects of LPS and dietary protein on rectal temperature, serum hormones, haptoglobin, plasma urea N and AA, and N balance were evaluated in 24 steers (250 +/- 2.8 kg of BW). Treatments were a 2 x 3 factorial of LPS (0 vs. 1.5 microg/kg of BW; -LPS vs. +LPS) and diets containing (DM basis) 1) 14.5% CP, 11.6% ruminally degradable protein (RDP), and 2.9% ruminally undegradable protein (RUP; CP14.5CON); 2) 16.3% CP, 13.4% RDP, and 2.9% RUP (CP16RDP); and 3) 16.1% CP, 11.2% RDP, and 4.9% RUP (CP16RUP). Diet RDP and RUP were altered using casein, fish meal, and corn gluten meal. Steers were adapted to diets (1.1 Mcal/kg of NE(g); DM fed at 1.8% BW) for 14 d and were infused (intravenously 1 mL/min) with LPS (in 100 mL of saline) on d 15. Rectal temperature and serum cortisol, prolactin, haptoglobin, and insulin increased, glucose initially increased and then declined, and serum thyroxine and triiodothyronine decreased for +LPS vs. -LPS steers (LPS x hour; P < 0.01). Serum IGF-I was less (P < 0.01) for +LPS vs. -LPS steers. Plasma urea N increased in response to LPS (LPS x hour; P = 0.02) and was greater for +LPS steers fed CP16RDP and CP16RUP vs. CP14.5CON, but greater in -LPS steers fed CP16RUP vs. CP16RDP and CP14.5CON (LPS x diet; P = 0.04). Plasma Met, Thr, Leu, Ile, Phe, Trp, Gly, Ser, Asn, and Tyr decreased, and plasma Ala increased in response to LPS (LPS x hour; P < 0.01). Plasma Orn initially increased and then decreased in +LPS vs. -LPS steers (LPS x hour; P < 0.01). No LPS x diet interactions (P > or = 0.15) occurred for DM, OM, NDF and N intake, fecal excretion, or apparent digestibility. Dietary DM, OM, NDF, and N intake, and retained N were less (P < 0.01) for +LPS than -LPS steers. Total N intake, apparent N digestibility, and retained N were greater (P < or = 0.05) for steers fed CP16RDP and CP16RUP vs. CP14.5CON. An LPS x diet interaction (P = 0.05) occurred for N retention (% N intake) because N retention was less for +LPS than -LPS steers when fed CP14.5CON, but not different between +LPS and -LPS steers when fed CP16RDP and CP16RUP. These results demonstrate that LPS infusion alters serum hormones, plasma AA, and N balance in cattle and imply that growing steers exposed to LPS may require greater dietary protein concentrations to account for altered intake and metabolic AA demand.