Previous experiments have demonstrated that significant increases in yield of Agaricus bisporus mushrooms were achieved by adding a micronutrient rich fertilizer, Micromax((R)), to the compost. This study was performed to determine the mineral(s) that are responsible for this yield improvement. An initial experiment determined that manganese was the mineral of primary importance; addition of 184 mg kg(-1) Mn increased yield by 10.8%, compared to the control. Subsequent experiments demonstrated that the addition of manganese to the compost has a stimulatory effect on mushroom yields. Significant yield increases, ranging from 9.6% to 11.8% (compared to the control), were observed as a result of manganese additions varying between 50 and 300 mg kg(-1). Also, data from the last set of experiments indicated that Micromax additions always resulted in greater yields indicating that micronutrients in Micromax, in addition to manganese, may be responsible for increasing yields. (c) 2005 Elsevier Ltd. All rights reserved.