Pretumorous lesions and lung and pleural tumours induced by asbestos in rats, Syrian golden hamsters and Macaca mulatta (rhesus) monkeys.

Asbestos samples were injected into the right pleural cavity of random-bred rats (20 mg thrice monthly). Milled commercial chrysotile induced pleural mesotheliomas in 65.5%, its dust in 46.3%, milled synthetic chrysotile in 2.4%, milled commercial magnesia-arphvedsonite in 77.1%, milled commercial anthophyllite in 41.4%, its dust in 64.9% and milled synthetic hydroxyamphibole in 54.5%. Intratracheal injections (10 mg twice monthly) of milled commercial chrysotile into Syrian golden hamsters induced lung tumours in 63% (malignant in 44.4%, benign in 18.5%), synthetic chrysotile in 23% (malignant in 7.7%, benign in 15.3%), commercial magnesia-arphvedsonite (5 mg twice monthly) in 80.8% (malignant in 46.2%, benign in 34.6%) and synthetic hydroxyamphibole (5 mg twice monthly) in 63.4% (malignant in 19.2%, benign in 44.2%). Benzo[a]pyrene adsorbed or added to asbestos dust stimulated asbestos-induced lung carcinogenesis in rats but had no influence on pleural carcinogenesis. Pretumorous lesions were found in lung tissue and in pleural mesothelium in all experiments in rats, hamsters and monkeys after instillations of asbestos into the lungs and pleural cavity. The morphological picture of the pretumorous lesions and tumours was similar in all experiments. Diffuse and focal hyperplasia and proliferation of epithelium-like and fibroblast-like mesothelial cells were observed. Benign (adenoma-like, fibrous) and malignant (carcinoma-like, sarcoma-like, mixed) mesotheliomas were also found.