Elaboration of new substrates for cultivating Pleurotus ostreatus (Jacq.) P. Kumm. based on degraded substrates from edible fungi cultivation We investigate the viability of commercially growing Pleurotus ostreatus (Jacq.) P. Kumm., second mushroom in importance cultivated in Spain, using the spent mushroom substrate remaining after the cultivation of P. ostreatus and Agaricus bisporus (J.E. Lange) Pilát. After the chemical and biological characterisation of the substrates, the qualitative and quantitative production parameters were evaluated in a P. ostreatus production cycle. Substrates based on combinations of Pleurotus spent substrate and spent Agaricus compost mixed in proportions of 9:1 and 8:2 (w/w) provided biological efficiencies of 36.0 and 39.7 kg/100 kg substrate, respectively, which is not significantly different from the values obtained with the commercial substrate used as control. However, the mushrooms obtained were smaller and had lower protein and ash contents. In principle, this kind of material could be incorporated in new formulations and methodologies with the advantage of lowering costs and diminishing the environmental impact of the wastes produced during mushroom cultivation.