A cell line from a female Microtus agrestis has been established in vitro, with most cells showing the normal female complement with two entire X chromosomes. Cloning of this line yielded stable cell strains, some of which showed deletions of the X chromosomes. Karyotypically, the deletions include (1) loss of the long arm of one X chromosome, (2) loss of one entire X chromosome, and (3) loss of one entire X chromosome together with loss of the long arm of the other X chromosome. In the last case, only the “functional” portion of the presumably active X chromosome remained. Radioautography has confirmed the actual loss of both constitutive and facultative heterochromatin from the X chromosomes in these clones. In addition, the parent cell line yielded a clone tolerant to 5-bromodeoxyuridine (BUdR) at 5 µg/ml. This cell strain retains thymidine kinase activity, however, as evidenced by uptake of thymidine, and by incorporation of BUdR into the