This study was carried out to determine the influence of ram horn hydrolysate (RHH) supplementation on the growth performance and slaughter traits of broilers. A total of 240 male broiler chicks (Ross 308), received from a commercial hatchery at I day of age, were allocated to four dietary treatments (H-0, H-1, H-2 and H-3 groups) in a completely randomized experimental design. Feed and water were offered ad libitum and lighting was continuous throughout the experimental period. The Ho group was fed with basal diet plus normal drinking water. The H-1, H-2 and H-3 groups were fed with basal diet plus 1%, 2% and 3% RHH-added water, respectively, in place of normal drinking water to meet the daily water requirements of broiler chicks from I to 28 days of age. All birds were housed in batteries from 1 to 21 days, and in grower broiler pens to 49 days. Feed intake and body weight gain were recorded weekly per pen. The average final body weights were 2524.9, 2601.2, 2441.7 and 2424.9 g for the H-0, H-1, H-2 and H-3 groups, respectively, and the difference was significant (P < 0.05). The average daily weight gains were 50.66, 52.25, 49.05 and 48.65 g, respectively, and the supplementation had no significant effect on this parameter. Daily feed consumptions were 96.18, 95.25, 91.70 and 90.75 g, respectively, and the difference between control and treatment groups was significant (P < 0.01). The feed conversion ratios (FCR) were, respectively, 1.79, 1.71, 1.73 and 1.81 (P > 0.05). At the end of the trial all birds were slaughtered to determine the slaughter traits. The hot carcass weights and yields were 1874.0, 1934.7, 1767.0, 1845.3 g and 75.78, 77.80, 75.00, 77.41%, respectively, and the difference was significant (P < 0.01). In addition, offal weights were determined and it was observed that there was no difference among the groups. Abdominal fat pad weights were similar in all groups. The H-1 group experienced more effects on some performance and slaughter traits than the control and the other RHH-treated groups, and the supplementation of diets with RHH at a level of 1% improved broiler performance. (c) 2005 Society of Chemical Industry.