The effects of urethan on RTG-2 and FHM cells were studied in vitro, by using the mitotic index, it was determined that 0.3 percent urethan caused an increase in the rate of cell division while higher concentrations (0.6, 0.9, 1.2, and 1.5 percent) caused either a decrease in the rate or a cessation of cell division. Concentrations of urethan higher than 1.5 percent killed the cells. The mitotic index data also indicated that epithelial cells continued to divide at a higher concentration of urethan than did the fibroblast cells. The morphological effects of urethan on the two cell lines were also investigated. These effects included vacuolization of the cytoplasm, lobed and enlarged nuclei, and in some cells the cytoplasm almost completely disappeared and the nucleus developed a thick membrane oround it so that the cells resembled small lymphocytes.

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