The effect of a commercial manure additive (Alliance) on ammonia (NH3) emissions was evaluated in commercial 1000-head grow-finish swine buildings over a six-month period. The test was conducted in two treated and two control buildings at a modern swine-finishing site consisting of nine identical buildings. Automatic spray application systems in the treated buildings intermittently sprayed the additive onto the surfaces of the below-floor manure storages. Ammonia concentrations were measured with a chemiluminescence analyzer at three location groups in each building over 7 or 12 min periods every 1.0 to 1.5 h. Pit fan airflow rates were measured continuously with impeller anemometers. Wall fan airflow rates were calculated from fan pressure/airflow curves and measured static differential pressure between indoor and outdoor air. Nearly 7,000 data subsets from 332 building-days of testing were obtained for comparing NH3 emission rates between control and treated buildings. The mean NH3 emission rate per AU (animal unit or 500 kg live weight) from the treated buildings (96.4 g/day(.)AU) was 24% (P < 0.05) lower than the control buildings. The volume of additive solution was sufficient to dilute the fresh manure by 20%, but the effect of dilution only on NH3 emission was not measured.