Feeding rumen-undegradable protein (RUP) is a practical strategy to supplement essential amino acids to high producing cows and reduce N losses to the environment. The objective of this study was to determine the effects of a novel RUP source (ADM) on performance and nutrient digestion by Holstein cows. A total of 18 Holstein cows [BW (653.0 ± 96.5 kg); DIM (123 ± 50 d)] were assigned to 1 of 3 treatments in a replicated 3 × 3 Latin Square Design with experimental periods 28d in length. Periods were divided in an adaptation phase (d 1 to 21), and measurement phase (d 22 to 28). Treatments were CON, cows were fed a control diet (16% CP, 70% RDP, and 30% RUP) without a RUP source; AMP, cows fed a diet with RUP source 1 (16% CP, 60% RDP, and 40% RUP); and ADM, cows fed a diet with RUP source 2 (16% CP, 60% RDP, and 40% RUP). Data were analyzed using the MIXED procedure of SAS. Two treatment orthogonal contrasts were used. Contrast 1 (CONT1): CON compared with ADM and Contrast 2 (CONT2): AMP compared with ADM. There were no treatment differences for either contrasts (CONT1 or CONT2) for DMI as a percent of BW, BCS, or fat-corrected milk (3.5%) (P > 0.10). Milk protein and lactose yields were higher for cows in CON (1.14 ± 0.02 kg/d and 1.78 ± 0.03 kg/d, respectively) than AMP (1.08 ± 0.03 kg/d and 1.68 ± 0.03 kg/d; P = 0.02 and P = 0.03, respectively; CONT1). However, milk lactose concentration and casein as a percentage of protein were higher for cows in ADM (4.75 ± 0.01% and 76.62 ± 0.17%, respectively) than AMP (4.70 ± 0.02% and 76.22 ± 0.18%; P = 0.01 and P = 0.03, respectively; CONT2). Cows in ADM (14.21 ± 0.4 mg/dL) had lower MUN concentration compared with cows in CON and AMP (15.88 ± 0.5 mg/dL and 16.08 ± 0.4 mg/dL; P < 0.05, CONT1, and P < 0.05, CONT2). Cows in ADM (0.36 ± 0.02mg/dL) had lower BHB plasma concentrations than CON and AMP (0.42 ± 0.02 mg/dL; P < 0.05, CONT1, and P < 0.05, CONT2).Cows in ADM had the lowest treatment BHB plasma concentrations on d 26 (0.34 ± 0.02mg/dL) compared with d 22 (0.38 ± 0.03 mg/dL). In conclusion, performance of cows in ADM was more similar to Cont than those of AMP and seem to have lower protein breakdown (i.e.; deamination) than cows in CON and AMP.