Many workers as well as the general public are exposed to glass fibers, which are among the most common man-made fibers. Information related to their genotoxicity and potential carcinogenicity is still limited. In this study, we investigated the ability of glass fibers to induce micronucleated and multinucleated cells in cultured Chinese hamster lung fibroblasts, the V79 cells. The induced micronuclei were further analyzed to determine the mechanism of micronucleus formation by staining the kinetochore with anti-kinetochore and fluoresceinated goat anti-human immunoglobulin G (IgG) antibodies. Three types of glass fibers (Manville 100 microfiber, Owens Corning AAA-10 microfiber, and Owens Corning general building insulation fiber) were studied. The results show that the two microfibers induced significant numbers of multinucleated and micronucleated cells in a concentration-related manner. Immunofluorescent staining demonstrated a significant doserelated increase in the proportion of kinetochore-positive micronuclei in cells treated with the two microfibers. These results indicate that the two microfibers are capable of inhibiting cytokinesis and are principally aneuploidogens. Unlike the two microfibers, the larger fibers neither induced micronuclei nor inhibited cytokinesis in V79 cells. Thus, the genotoxic potential of glass fibers in V79 cells may be related to their size.