This study evaluated the efficiency of lysine utilization by growing steers. Five ruminally cannulated Holstein steers (165 kg ± 8 kg) housed in metabolism crates were used in a 6 × 6 Latin square design. Data from a sixth steer was excluded due to erratic feed intake. All steers were limit-fed (2.46 kg DM/d) twice daily diets low in RUP (81% soybean hulls, 8% wheat straw, 6% cane molasses, and 5% vitamins and minerals). Treatments were: 0, 3, 6, 9, 12, and 15 g/d of L-lysine abomasally infused continuously. To prevent amino acids other than lysine from limiting performance, a mixture providing all essential amino acids in excess was continuously infused abomasally. Additional continuous infusions included 10 g urea/d, 200 g acetate/d, 200 g propionate/d, and 50 g butyrate/d to the rumen and 300 g glucose/d to the abomasum. These infusions provided adequate ruminal ammonia and increased energy supply without increasing microbial protein supply. Each 6-d period included 2 d for adaptation and 4 d for total fecal and urinary collections for measuring N balance. Blood was collected on d 6 (10 h after feeding). Plasma urea-N was decreased (P = 0.01) by lysine supplementation, but plasma glucose was unaffected (P = 0.73). Diet DM digestibility was not altered (P = 0.94) by treatment and averaged 72.2%. Increasing lysine supplementation from 0 to 9 g/d decreased urinary N excretion (P < 0.01) from 32.3 to 24.3 g/d with no further reduction when more than 9 g/d of lysine was supplied. Changes in total urinary N excretion were due to changes in urinary urea-N, which decreased (P < 0.01) from 21.6 to 14.8 g/d as supplemental lysine increased from 0 to 9 g/d. Urinary ammonia-N was not affected by treatment (P = 0.48). Increasing lysine supply from 0 to 9 g/d increased (P < 0.01) N retention from 20.7 to 29.9 g/d with no further increase observed beyond 9 g/d of lysine. Break-point analysis estimated maximal N retention at 9.0 g/d supplemental lysine. Over the linear response surface of 0 to 9 g/d lysine, assuming that retained protein is 6.25 × retained N and contains 6.4% lysine, the efficiency of lysine utilization for protein deposition was 40%.