Chicken feather keratin was treated with lime (calcium hydroxide) to obtain a liquid product rich in amino acids and polypeptides that can be used as an animal feed supplement. The effect of treatment conditions and the properties of the soluble keratin were studied. At high temperatures (150 degrees C), 80% of feather keratin was solubilized within 25 min, whereas a relatively longer reaction time (300 min) is needed at moderate temperatures (100 degrees C). After 3 h of hydrolysis at 150 degrees C, 95% of feather keratin was digested. For the recommended conditions (100 degrees C, 300 min, and 0.1 g Ca(OH)(2)/g dry feather), after lime treatment, about 54% of calcium can be recovered by carbonating. In rumen fluid, ammonia production from soluble keratin was similar to that of soybean and cottonseed meals and was greatly less than that of urea, indicating that no ammonia toxicity will result from cattle being fed soluble keratin. (c) 2005 Elsevier Ltd. All rights reserved.