Fibrogenic properties of two dusts of synthetic hydrated amorphous silicas, Ze-O-Sil (French production) and Arsil (Polish production) were studied. Experimental silicosis was induced by intratracheal administration to rats of a single dust dose (50 mg in 0.6 ml NaCl). Fibrogenic properties were tested 3, 6, 9 months after administration of the dust. X-ray diffraction and chemical tests did not reveal any form of crystalline. Si02 in the composition of the investigated dusts. The dusts tested as compared with quartz, were characterized by a great solubility up to 211 mg/l, which made the dust excretion from the lungs easy; after 6 month-experiment approximately 1.2 mg of Arsil dust and about 28 mg of quartz dust residue was found. Fibrogenic properties of Arsil dust, represented by wet lungs weight increase and hydroxyproline content in lungs, were more pronounced than those of Ze-O-Sil. Hydroxyproline content 3 months after administration of Arsil, Ze-O-Sil, TiO2 and weak quartz amounted to 7.3 mg, 5.1 mg, 3.8 mg and 6.0 mg, respectively. Histological and ultrastructural investigations demonstrated that disseminated, multifocal granulomas were the basic reaction to both dusts; no clear histological signs of cytolytic action on the cells of dust granulomas were found. In the lungs no silicotic nodules or degeneration of changes were revealed. Neither biochemical tests nor histological examinations revealed a progressive development of fibrous connective tissue. In result of the studies the authors suggest 2 mg/m3 as a MAC value for Arsil dust instead of 10 mg/m3--the value which has been hitherto used.