The carcinogenic and fibrogenic activity of four samples of synthetic amphibole asbestos with different chemical structure was examined in white rats. Pleural mesotheliomas were found: in 11 out of 27 rats (37.8%) treated with magnesium asbestos; in 4 out of 24 animals (6.7%) for nickel asbestos; 13 out of 22 (59.1%) for cobalt asbestos; and in the experiment with asbestos where germanium was substituted for silicon pleural mesotheliomas were observed in 2 out of 55 rats (3.6%). No tumours of this kind were found in the control group. As revealed by the study results, all the examined samples of synthetic amphibole asbestos exhibited carcinogenic potentials. Correlation between the carcinogenic and fibrogenic activity could also be observed. The replacement of silicon with germanium produced considerable decrease in the carcinogenic and fibrogenic potentials of asbestos.