A line of C3H mouse embryo cells highly sensitive to postconfluence inhibition of cell division, designated C3H/10T1/2, has been established, and a clone from this line (clone 8) has been characterized at early and late passages (200 to 450 days in culture). The cells of clone 8 are approximately 1730 cu µm in volume, their plating efficiency is 12 to 30%, their average generation time is 15.5 hr, and their saturation density is 2.9 to 3.8 × 10⁴ cells/sq cm. The cell survival is 30% after freezing in 10% dimethyl sulfoxide and storage in liquid nitrogen. The cells of this clone are negative with respect to the spontaneous expression of C-type RNA murine viruses and viral antigens. Tests for mycoplasma contamination are negative. All the cells of this line are aneuploid with a stable mode of 81 chromosomes (40 to 60% of cells). Tests for tumorigenicity at all passages were negative. No spontaneous transformation in vitro has been observed in the stock cultures transferred on a regular schedule.