Calculation of protein requirements with a metabolizable protein (MP) system yielded estimates of protein needs by finishing cattle that agreed more closely with current industry practices than did calculation based on the factorial method. The difference between the MP system and the factorial method was primarily a result of accounting for microbial N needs in the MP system. Reasons for production responses to CP levels that are greater than those calculated by the factorial method include increased accretion of protein by rapidly growing, implanted cattle, particularly during the initial phase of the finishing period, alleviation of a microbial N deficiency, and ruminal and systemic effects of ruminally degraded N on acid-base balance of beef cattle fed rapidly fermented, high-grain diets. Reasons for production responses to supplemental CP need further research.