An experiment was conducted to assess the effects of a high-protein corn product (56% crude protein; CP) relative to other sources of protein on the lactation performance of dairy cows. Twenty-four Holstein cows (620 ± 47.7 kg of body weight, 98 ± 34 d in milk, 2.28 ± 0.46 lactations; mean ± SD) were randomly assigned to treatment sequence in a replicated 4 × 4 Latin square design balanced for carryover effects. Cows were individually fed one of 4 diets with a different protein concentrate source during each 28-d period, including: soybean meal (SBM), high-protein corn product (HPCP), soybean meal with rumen-bypass protein (SBMBP), and canola meal with rumen-bypass protein (CANBP). Diets were formulated for equal concentrations of CP (17%) and balanced to meet lysine and methionine requirements. The SBM diet was formulated to provide 5.7% rumen-undegradable protein (RUP), while SBMBP and CANBP diets were formulated for 6.8% RUP to match HPCP. Data were analyzed with mixed models accounting for diet, period, and cow (random), and differences between treatment means were determined using Tukey’s HSD. The CANBP diet increased dry matter intake (P < 0.01) compared with SBM and HPCP (28.9 vs. 27.6 and 27.3 ± 0.46 kg/d). Treatment affected milk yield (P < 0.001), as SBMBP and CANBP increased yield compared with SBM (42.0 and 42.4 vs. 39.9 ± 1.79 kg/d), but HPCP decreased milk yield compared with other treatments (37.4 ± 1.79 kg/d). The SBM diet increased milk protein concentration relative to HPCP (3.08 vs. 3.00 ± 0.04%; P < 0.01). Both SBM and SBMBP increased milk urea nitrogen (MUN) compared with CANBP (11.7 and 11.6 vs. 10.8 ± 0.34 mg/dL, P < 0.01), but HPCP decreased MUN compared with other treatments (10.0 ± 0.34 mg/dL). The HPCP diet reduced protein, lactose, and solids nonfat yields compared with all other treatments (P < 0.001). There was no evidence of treatment effects on body weight (P = 0.89) or body condition score (P = 0.19) change. In conclusion, the HPCP diet reduced milk and components yields and MUN, possibly due to lower amino acid content as a percentage of crude protein, lower protein digestibility, or both.