Abstract: An improved substrate for mushrooms that is essentially cellulose-free comprising a major portion of grain and minor portions of nutritional supplements. The grain is partially sterilized by boiling in order to kill bacteria, cooled in order to induce germination of the heat resistant spores, and steam sterilized before the germinated spores have matured sufficiently to create new spores. The substate can be used to grow fungi, including mushrooms, especially shiitake.