Everywhere in the world button mushroom is cultivated on heat-treated horse manure or on a compost mixture of straw, horse manure and chicken manure. In the ZKI trials continued on straw which seemed to be a simple and cheap substrate. To improve yield the N-content of the straw was increased by mixing it with wheat bran, alfalfa flour and ProMycel. Enriching agents were added to the dry heat-treated straw (100 "C) at 1, 2, 3 weight per cent. All the 3 agents increased yield as related to control. The highest yield was obtained with ProMycel.