Cell lines grown from brain tissue obtained during diagnostic biopsies of two patients with subacute sclerosing panencephalitis (SSPE) stopped dividing after 12 to 14 passages. They were maintained in this stationary phase by refeeding with fresh medium at weekly intervals. After 5-7 weeks, clusters of surviving cells exhibited luxuriant growth, nuclear pleomorphism, and aneuploid karyotypes. Isoenzyme patterns and mixed agglutination confirmed the human origin of the cells. No evidence of infection with SV40, or viruses other than the measles-like agent previously reported in such cells in SSPE, was detected. The possible mechanisms of this transformation are discussed. © 1969.