The effect of Alliance, a manure additive, on air quality was tested in four commercial grow-finish swine buildings over a period of six months. The test was conducted in two treated and two control buildings. The additive was sprayed onto the surface in the manure pit with an automatic spraying system. Ammonia concentrations were measured at three locations in each building over a 7- or 12-minute period every 60 or 90 minutes. Pit chimney ventilation rates were measured with full-size airflow sensors. Tunnel wall fan ventilation rates were calculated with fan operation and static differential pressure between indoor and outdoor air. A total of 332 days that included 6 944 data subsets were obtained for comparing ammonia emission. The average ammonia emission rate per animal unit from the treated buildings was 24.2% lower than the control buildings.