Two experimental biofilters were used to determine whether an airstream treated by a biofilter could be distinguished from ambient farmyard air. Odourous air (mean of 958 OU measured using olfactometry) from a nearby hog barn passed through the first biofilter, but "clean farmyard" air (mean of 34 OU) passed through the second biofilter. Six different biofilter media mixtures were formulated using three types of compost (yard-waste compost, grocery-waste compost, and poultry-manure compost) and three bulking agents (woodchips, hemp hurds, and unchopped wheat straw) blended in a 50%/50% mixture (by mass). Complete treatment of the airstream was achieved, based on outlet H2S levels <11 ppb, for mixtures containing yard-waste or grocery-waste compost combined with either woodchips or hemp hurds. Outlet odour concentrations ranged from 22 to 51 OU for the odourous-air biofilter and from 26 to 51 OU for the clean-air biofilter. Thus, the residual odour of an airstream following passage through a biofilter bed consisting of compost and a bulking agent cannot be distinguished from ambient farmyard odour using olfactometry.