The effect of castration on endogenous proteinase activity and myofibrillar protein turnover was investigated in cattle. Six each of MARC III composite bulls and steers weighing approximately 210 kg were given ad libitum access to a typical growing diet. At 0, 42, 84, 126, and 168 d, two consecutive 24-h urine samples were obtained. Urine was analyzed for N tau-methylhistidine (N tau MH) and creatinine. Following slaughter after 170 d on feed, a longissimus muscle sample was removed immediately from each carcass for quantification of mu-calpain, m-calpain, calpastatin, cystatin(s), cathepsin B, and cathepsin B + L activities. Bulls were heavier (P < .05) at 126 and 168 d and more efficient (P < .05) in conversion of feed to gain at 84 and 168 d than were steers. Compared with steers, bulls excreted less (P < .05) N tau MH at 84, 126, and 168 d and displayed lower (P < .05) fractional degradation rates (FDR) at all sample times. No differences (P > .05) in calpain or cathepsin activities were observed between bulls and steers. However, muscle from bulls had greater (P < .05) activities of calpastatin and cystatin(s) than that from steers. A negative relationship existed between d-168 FDR and calpastatin (r = -.72; P < .05) and cystatin (r = -.62; P < .05) activities. These results indicate that decreased FDR of skeletal muscle from growing bulls contributes to their greater efficiency of growth and could be related partially to cystatin-mediated cathepsin activity and(or) calpastatin-mediated calpain activity.