This study was carried out to evaluate and compare the effect of feeding antibiotics on growth performance and rumen characteristics of Holstein bulls. The antibiotics used in this experiment were monensin and lasalocid as ionophores and bambermycin. The results obtained were summarized as follows: 1. The average daily gain of bulls fed antibiotics was increased by 2.4∼7.9% compared with control(P$lt;.05). Dry matter intake of bulls moderatery decreased with the addition of antibiotics except in C treatment (bambermycin group). 2. The feed efficiencies of bulls fed antibiotics were increased by 6.1∼6.7% compared with the control. 3. Digestible crude protein and total digestible nutrient requirements per ㎏ body weight gain decreased 6.07.8% by the addition of antibiotics. 4. Contents of total volatile fatty acid and propionate in the rumen were higher in bulls fed antiobiotics than in control. The acetate/propionate ratio of bulls fed antibiotics decreased compared with control. 5. Numbers of E. coil colony units in feces decreased with addition of antibiotics.