A novel approach to utilizing whey permeate, the cultivation of mycelia of the edible mushroom Pleurotus ostreatus, is introduced. Response surface analysis (RSA) was successfully applied to determine the combination of substrate concentration, temperature and pH that would result in a maximal mycelial extension rate under solid state cultivation. The conditions to maximize the mycelial extension rate were predicted to be 44 g lactose l(-1), pH 6.0 and 24.2 degrees C. Subsequent verification of these levels agreed with model predictions.