Forty-six Thoroughbred and Quarter Horse yearlings were used in two experiments designed to determine the amount of lysine in the concentrate necessary to promote maximum growth when fed the Coastal bermudagrass (Cynodon dactylon) hay provided at 1% body weight (BW)/day. In the first experiment, a 196-day trial, supplemental protein sources and analyses of the concentrates compared were: (1) soybean meal (SBM; 15.4% crude protein, .70% lysine); (2) SBM + .2% lysine (15.7% crude protein, .81% lysine), and (3) brewers dried grains + .2% lysine (15.9% crude protein, .59% lysine). Concentrate intake averaged 1.67% BW/day, and hay intake averaged .94% BW, for a total of 2.61% BW/day. Protein and lysine intakes (grams/day) for horses fed the three diets were, respectively: (1) 1,106, 48; (2) 1,091, 52 and (3) 1,077 and 40. Growth and efficiency responses for animals on treatments 1, 2 and 3 were, respectively: weight gain - 125, 123 and 114 kg; height gain - 9.5, 9.3 and 8.1 cm; girth gain - 21.6, 19.0 and 17.0 cm (P greater than .05); length gain - 14.5, 13.7 and 14.9 cm. and feed/gain - 15.3, 15.0 and 16.0. In the second experiment, a 140-day trial, SBM-based concentrates provided (1) 16.1% crude protein and .67% lysine, (2) 13.6% crude protein and.51% lysine and (3) 14.2% crude protein and .64% lysine. Concentrate averaged 1.99% BW and hay intake averaged .92% BW, for a total of 2.91% BW/day. Protein and lysine intakes (grams/day) for horses fed the three diets were (1) 1,432, 59; (2) 1,195, 46; and (3) 1,182, 53. Growth and efficiency responses for animals given the three treatments were: weight gain - 101, 87 and 101 kg (P greater than .05); height gain - 5.7, 6.0 and 6.4 cm; girth gain - 14.8, 12.2 and 16.3 (P greater than .05); length gain - 10.9, 10.9 and 11.9 cm, and feed/gain - 16.5, 17.9 and 15.0 kg (P greater than .05). No significant sex effects were detected. Results suggest that at least 1.9 g lysine/Mcal digestible energy is required to support maximum growth of yearling horses.