Bull calves were castrated using a Burdizzo to determine the effect of age at castration on physiological and immunological stress indices. Five age groups were studied: 1.5, 2.5, 3.5, 4.5, and 5.5 months of age, or the calves were sham castrated at 5.5 months of age to serve as a control. The results indicated that calves that were castrated at 5.5 months of age had significantly higher peak plasma cortisol responses than intact calves. All calves castrated at a younger age had lower peak responses, with bulls castrated at 1.5 months having the lowest. For the first 3 hours after castration, the integrated cortisol responses were three-fold greater in the 5.5 month old than intact calves. For the following 9 hours after castration there were no significant differences in integrated cortisol responses among the treatments. The calves castrated at 5.5 months of age had significantly greater concentrations of plasma haptoglobin and fibrinogen than intact calves. These concentrations were significantly reduced in calves castrated at 1.5 and 2.5 months of age compared to the 5.5-month castrates. On the first day following castration, there was a suppression of phythohaemagglutinin-induced in vitro interferon- production in calves castrated at 5.5 months of age compared to intact calves. All castrates had increased scrotal circumferences on days 1 and 7. There was a reduction in swelling in the 1.5 month olds compared to the 5.5 month old castrates. Castration did not affect growth rate among the calves. The authors concluded that castrating calves at 1.5 months of age causes less physiological stress and inflammation than castrating at 5.5 months of age.