Asbestiform minerals, namely serpentine (chrysotile) and amphiboles (tremolite–actinolite) as well as others of fibrous habit (sepiolite) from Argentine deposits were characterised to assess their biological hazard. Materials currently used as asbestos substitutes were also assessed (glass fibre and ceramic fibre). Studies with laboratory animals were conducted. Sixty mice (30 females and 30 males), 8 weeks old, were used following the good practices in the care and handling of laboratory animals. They were split into six groups of ten mice each (5 females and 5 males), designated as control, tremolite, glass fibre, chrysotile, sepiolite and ceramic fibre, and were exposed by inhalation to these previously ground materials. The animals were examined throughout the experiments to see how they responded to the inhaled substances to establish the effects of inhalation at the time of exposure and once they had been killed. The respiratory tract (trachea, bronchi and lungs) of the animals exposed to the different materials was examined in tissue sections. Differences in the amount of connective tissue in the lungs and the presence of alveolar macrophages were observed in the animals exposed to tremolite, chrysotile and sepiolite, as compared to the control group. It was concluded that, of all the materials used, amphiboles caused greater damage and an incipient development of carcinogenesis. The other ones, especially chrysotile, produced connective tissue thickening.