Amino acid supplementation of practical-type rations containing relatively high levels of blood and feather meals, and its effect on broiler chicks' performance were investigated. In experiment 1, four diets were tested: a corn-soybean meal diet and diets containing 10% ring-dried blood meal, 6% hydrolyzed feather meal and a combination of the two. Diets were supplemented with the deficient amino acids. Results showed no significant differences among the four diet treatments with respect to weight gain or feed efficiency of chicks from 1 to 4 weeks of age. In experiment 2, further assessment of amino acid addition to blood-feather meal diets was done. Methionine was added to the basal diet at 0 or 0.21%, lysine at 0 or 0.2% and isoleucine at 0 or 0.33% in a 2 x 2 x 2 factorial arrangement. Growth and feed efficiency of chicks fed the blood-feather meal basal diet were improved by the addition of 0.21% methionine or 0.33% isoleucine. These parameters reached maximums when both methionine and isoleucine were added. Lysine addition at 0.2% had no significant effect on weight gain or feed efficiency. Experiment 3 was conducted to test higher levels of supplemental methionine and isoleucine in blood-feather meal diets. Methionine and isoleucine were added to the basal diet at 0, 0.2 and 0.4% in a 3 x 3 factorial arrangement. The addition of 0.2% methionine improved weight gain and feed efficiency, but 0.4% supplemental methionine resulted in no further improvement. Addition of 0.2% isoleucine improved weight gain and feed efficiency, but 0.4% isoleucine reduced weight gain and feed efficiency. Addition of methionine (the first limiting amino acid) and isoleucine (the second limiting amino acid) reduced the high levels of plasma leucine in chicks fed the basal diet (2.4 to 2.5%);Two additional experiments were conducted to test the effect of sulfur amino acid supplementation to blood-feather meal diets. In the first experiment methionine and cystine were added to the basal diet (0.3% methionine and 0.48% cystine) at 0, 0.1, 0.2, 0.3 and 0.4% and 0 and 0.1%, respectively, in a 2 x 5 factorial arrangement. A corn-soy based diet was used as a positive control diet. In a second experiment 0, 0.1, 0.2, 0.3 and 0.4% supplemental methionine and 0, 0.1 and 0.2% supplemental cystine were added to a milo, blood-feather meal basal diet (0.2% methionine and 0.48% cystine) in a 3 x 5 factorial arrangement. In the first experiment, 0.2% added methionine or 0.1% added cystine improved weight gain and feed efficiency of chicks fed the blood-feather meal diet. In the second experiment, weight gain increased linearly with increasing level of methionine. Maximum weight gain and feed efficiency, however, were observed when 0.1 or 0.2% supplemental cystine were added with 0.4% methionine, and then declined with higher levels of methionine supplementation. Plasma methionine-cystine concentrations in both experiments showed no evidence of a cystine-methionine antagonism.