Abstract: A cellulosic starting material, substantially devoid of nitrogen values is moistened to bring its water content to at least 50%, pasteurized by steam treatment during which the temperature is increased to a maximum of about 70.degree. C and maintained at such temperature for about 12 hours during which time a relative humidity of about 100% is likewise maintained whereby the starting material is digested. The material is cooled to about 55.degree. C to proliferate thermophilic metabolite producing microorganisms. This temperature is maintained for about 36 hours and is then lowered to about 40.degree.-45.degree. C and maintained at that level for about 24 hours whereby another group of microorganisms is proliferated. The substrate so produced is cooled to 25.degree. C or less. The process provides the substrate with metabolites that protect it against microorganisms which compete for substrate carbon sources with the macrofungi to be cultivated on the substrate.