The paper is focused on the research of ecotoxicological properties of mortar prisms produced with partial cement replacement by ash from energy recovery of municipal waste. Two types of ash were used: ash from incineration and ash from municipal waste gasifi cation. According to the Waste Catalogue, ash is considered other waste, which is non-hazardous and nowadays it is predominantly landfi lled. Negative results of standardized biotests are inevitable precondition for the use of ash for construction products. The results from both biotests (acute toxicity test on aquatic organisms Daphnia magna and growth inhibition test of higher cultivated plants Sinapis alba) confi rmed suitability of cement replacement by ash from energy recovery of municipal waste. Environmental safety of produced mortar prisms is different. Recommended replacement of cement with ash, obtained from municipal waste gasifi cation, is 10% and with ash gained from incineration is 15%. The use of this type of waste in construction industry will lead to the decrease of landfi lled waste. Due to the replacement of cement with waste (from industrial branches) natural resources of raw materials used in the process of cement production are saved.