Six types of man-made fibers were administered intratracheally (2.0 mg/animal each a week, for 5 weeks; total 10 mg/animal) to female Syrian hamsters that were observed histologically for 2 years after administration. The fibers were rock wool [average diameter (D) = 6.1 microns, average length (L) = 296 microns], fiberglass (D = 0.65 microns, L = 16.8 microns), potassium titanate fiber (D = 0.36 microns, L = 7.17 microns), calcium sulfate fiber (D = 1.0 microns, L = 17.8 microns), basic magnesium sulfate fiber (D = 0.45 microns, L = 22.4 microns), and metaphosphate fiber (D = 2.38 microns, L = 64.1 microns). Tumors were observed in hamsters that had received basic magnesium sulfate fiber (9/20), metaphosphate fiber (6/20), calcium sulfate fiber (3/20), and fiberglass (2/20) but not in the control, rock wool, or potassium titanate fiber groups. The primary sites of the tumors were not only in the pleural cavity but also in the intracelial organs, kidney, adrenal gland, bladder, and uterus. Only a few of the tumors were identified as mesotheliomas by histological examination. In addition to neoplastic lesions, fibrosis, pleural thickening, and chronic inflammatory changes in the lungs were observed in the hamsters, but these changes appeared too mild to foster a pneumoconiosis such as asbestosis.