A pasteurized, non-composted substrate (basal mixture) consisting of oak sawdust (28%), millet (29%), rye (8%), pear (8%), alfalfa meal (4%), soybean flour (4%), wheat bran (9%), and CaCO3 (10%) was adapted from shiitake culture to produce the common cultivated mushroom (brown; portabello), Agaricus bisporus. Percentage biological efficiency (ratio of fresh mushroom harvested/oven-dry substrate weight, %BE) ranged from a low of 30.1% (when wheat straw was substituted for sawdust) to 77.1% for the basal mixture. Special, high gas-exchange bags were required to optimize mycelial growth during spawn run. Our formula may allow specialty mushroom growers to produce portabello mushrooms on a modified, pasteurized (110 degreesC for 20 min) substrate commonly used for shiitake production without the added expense of compost preparation. (C) 2001 Elsevier Science Ltd. All rights reserved.