Two substrates, a non-composted grain spawn substrate and a traditional composted substrate, each covered with peat-based casing that contained varying amounts of activated carbon (AC) and each receiving different heat-treatment durations, were tested for Agaricus bisporus Mushroom production. The amounts of AC were 0, 5, 10, 15, and 20% v/v, and the heat treatments were 0, 60, and 180 min at 121 degrees C and 103.4 kPa. Overall, the addition of AC up to 10-15% of casing for a grain spawn substrate increased mushroom yield. However, the addition of AC to the casing for compost substrates had no significant effect on yield, whereas heat-treating the casing increased yield. The onset of fruiting was retarded in grain spawn treatments not receiving AC with heat-treatment durations of 60 and 180 min, whereas this effect was not as apparent for the compost substrates. On average, mushroom yield was greater for the grain spawn substrate (366 g) than for compost substrate (287 g). For grain spawn substrate, the results show that the addition of AC ranging from 5% to 10% was adequate for maximum mushroom production. (C) 2009 Elsevier Ltd. All rights reserved.