An experiment was mounted at a large fattening piggery in Slovenia, during which different groups each of 600 weaned pigs were housed and grown in different fattening roams for 60 days (from age 24 days to age 84 days). All rooms were mechanically ventilated and had fully-slatted floors with slurry channels beneath. De-Odorase(R)dagger+ was added to the diet of the treated groups of pigs at the rates recommended by the suppliers, e.g. for grower feed, 65 g/t. De-Odorase(R) was also added daily (3.2 g suspended in 10 l of water) to the upstream ends of the slurry channels. Once a week, the ammonia concentration in the air inside each fattening room was measured at each of four positions, using Drager tubes, and the ventilation rate was computed from the cross-sectional area of each fan outlet and the average velocity of the air emerging from each. Also, air samples were taken from immediately outside three of the fans sewing each room, and their odour concentrations were measured by olfactometry. Hence, weekly values of emission rates of both ammonia and odour could be calculated. Ammonia concentration was significantly lower (P = 0.001) in the De-Odorase(R)-treated buildings than in the control buildings. The average reduction in ammonia concentration over a 7 week period was 26%. Ammonia emission rate was also reduced by 26% through the use of the additive. This reduction was significant at P = 0.048. Neither odour concentration nor odour emission rate was significantly reduced through the use of the additive. The additive did not significantly influence the rate of live-weight gain by the pigs.