This paper presents results of biodeterioration of Mater-Bi YI01U under natural weather conditions in compost with sewage sludge. The tests were performed at a waste treatment plant and - for comparison - under laboratory conditions at stable temperature. The compost pile was prepared outdoors at a waste treatment plant and consisted of active sludge, burnt lime /CaO/ (to ravage pathogenic bacterium and eggs of parasites) and straw. The characteristic parameters of compost, such a dry mass of compost, pH of compost and activity of dehydrogenases were determined. Changes in weight and morphology of Mater-Bi YI01U samples were tested during the experiment. The obtained weight loss results were compared with that from Weight Loss Under Controlled Composting Condition test method, which simulates the biodegradation behaviour of a material under composting conditions. According to this test method, Mater-Bi Y Class lost more than 90% of weight after 4 months when disposed of in the standard environment under controlled conditions, but in the investigated natural, weather-dependent, composting conditions this material lost about 20% of weight after the same time of incubation.