TECHNOLOGY FOCUS - BIOLOGY - Preferred Mixture: The mixture comprises (wt.%) either:(a) horse manure (75 - 85), chicken manure (10 - 15) and gypsum (3 - 8); or(b) straw (60 - 65), chicken manure (30 - 35) and gypsum (3 - 6).The moisture content of the mixture is 70 - 80%.Preferred Process: The microbial conversion is carried out at 60 - 100 degrees Centigrade and the compost is allowed to cool to ambient temperature before step (iii). Step (iii) is carried out at 50 - 60 degrees Centigrade. The pasteurized compost is cooled to 40 - 45 degrees Centigrade for 4 - 6 days and then to 20 - 30 (preferably 25 - 28) degrees Centigrade. The spawn is pre-cultured on the compost under normal spawn-running conditions. The compost is inoculated with 1 - 25 (preferably 6 - 15) % (vol./wt.) of spawn. The culturing of spawn is carried out in a 3 - 10 days to produce the substrate.Preferred Spawn: The spawn is Agaricus bisporus, Agaricus bitorquis, Lentinus edodes, Coriolus versicolor or Pleurotis species. EXAMPLE - Spawn on compost was produced in a pre-culturing step which was carried out in 15 days at 25 degrees Centigrade under anaerobic conditions and at saturated humidity in tunnel starting from phase 2 compost, being freshly inoculated with Sylvan Somycel 512 (RTM; grain spawn) (8 liter/ton phase 2 compost). Phase 2 compost was inoculated with the pre-cultured spawn at inoculation degrees (wt.%) of 1, 5 and 10, and the pre-cultured spawn was thoroughly mixed with the compost at 19 degrees Centigrade, cultured for 15, 8 and 6 days respectively. The compost was fully colonized and the spawn-run compost was harvested and used for mushroom cropping following a standard procedure. It was found that the mushrooms were of very high quality in every instance. In comparative process, for the production of a substrate for culturing mushrooms, phase 2 compost was inoculated with Sylvan Somycel 512 (RTM; 7 liter per ton phase 2 compost). The same culturing procedure was carried out as in the test process, but the culturing time was 13 days under conventional conditions. It was found that the mushrooms were of very high quality.