Sixteen Holstein and Brown Swiss cows, 112 ± 45 days in milk, were assigned by stage of lactation to a replicated 4 x 4 latin square experimental design with three-week periods. Treatments A, B, C and D were based on pelleted concentrates (CT), theoretically equal in contents of nitrogen (18.0 to 18.3% crude protection) and energy (2.6 Meal NEL/kg) and containing, respectively, 0, 2, 4 and 8% of SynerMax®3 (coproduct of antibiotic production). These were fed at a rate of 1 kg CT/2 kg milk (recalculated weekly) in three daily meals, along with limited access to long hay and nighttime grazing at four cows per hectare on unfertilized grass swards. Mean results for A through D were, respectively: daily CT intake, 10.64, 10.84, 10.25 and 10.57 kg; daily milk yield, 21.32, 21.91, 21.63 and 20.97 kg; milk fat content, 2.76, 2.75, 2.52 and 2.76%; protein content, 3.06, 3.02, 3.02 and 3.04%; ratio of milk to CT (M/CT), 2.01, 2.02, 2.11 and 1.98. Significantly different were the higher CT intake with 2% SynerMax® (B) vs. 4% and 8% (C + D) (0.43 kg difference, P < 0.05), and the higher M/CT with 4% than with 8% SynerMax® (C vs. D) (0.13 kg difference, P < 0.01). Incorporation of SynerMax at levels of 2 and 4% in pelleted CT assured good animal acceptance and had no adverse effects on lactational performance.