A high concentration of silica dust in the air many workstands in ceramic plants is a serious risk for the health of exposed workers. It can cause the development of silicosis. The aim of this paper is to evaluate a fibrogenic activity of dusts containing 98% crystalline silica. The ceramic masses have about 25% of Quartz. The samples of 2 lode quartz and 4 sand quartz originated from different mines in Poland were used for the experiment. Their physical properties were analysed by X-ray diffraction and SiO2 content was determined by colorimetry. Their biological aggresiveness was characterised by wet weight, hydroxyproline content, lipid level in the lungs and wet weight of mediastinal lymph nodes of rats which were intratracheally given of the dusts. The results show that the tested dusts have a moderated fibrogenic activity. At short time (3 months) after their administration the fibrogenic changes in lungs were different depending on the kind of dusts, but at the 6th month of the experiment they did not differ at all. The exposition of workers to silica dusts is a great occupational hazard.