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This work presents the results of the test performed on rats to evaluate the carcinogenic effect of 4 synthetic amphiboles compared to that of the natural amphibole--crocidolite. The dose of the magnesium amphibole (Na2Mg6Si8(OH)2) administered to the animals contained 240 x 10(6) respirable fibres; the corresponding value for the nickel amphibole (Na2Ni6Si8O22(OH)2) was 339 x 10(6), for the cobalt amphibole (Na2Co6Si8O22(OH)2)--1000 x 16(6) for the geranium amphibole (Na2Mg6Ge8(OH)2)--250 x 10(6), and of the natural crocidolite amphibole (Na2Fe2Fe3Si8O22(OH)2) x 380 x 10(6) respirable fibres. The control animals (rats) received physiological NaCl solution. The number of peritoneal mesotheliomas following single intraperitoneal administration of 20 mg of the dust was adapted to be the measure of the carcinogenic activity of the dust. 3 synthetic (magnesium, cobalt and nickel) amphiboles and crocidolite caused development of malignant peritoneal mesothelioma in 11.1% to 71% rats. The results show that there is a relationship between the chemical composition of the synthetic amphiboles and their carcinogenic effect. Out of 4 investigated synthetic amphiboles, the magnesium amphibole, which contained magnesium and silicon, displayed most severe carcinogenic effect. The synthetic amphiboles containing either silicon and cobalt or silicon and nickel displayed 8.3 and 6.2 times weaker ability to induce peritoneal mesothelioma.