The molecular weight of single-stranded DNA isolated from human fibroblasts decreased in phase III by comparison with phase II. Mouse fibroblast DNA isolated during the growth crisis had a decreased molecular weight compared to the initial DNA. Established mouse cells recovered this high molecular weight DNA. Cells treated with caffeine during the growth crisis did not survive while established cells were resistant to the same conditions of caffeine treatment. A DNA repair process may play a role in establishing a permanent cell line.