Two aneuploid cell lines which differ in their inherent sensitivity to ionizing radiation and chemotherapeutic agents were established concurrently from a single tumor specimen obtained from a patient with glioblastoma. M059J cells are approximately 30-fold more sensitive to radiation than are M059K cells (surviving fractions at 2 Gy were 0.02 and 0.64, respectively). This relative difference in radiation sensitivity has remained a stable feature of the cell lines during 2 years in continuous culture. In addition, cells of the M059J line are more sensitive than those of the M059K line to the cytotoxic effects of bleomycin, N,N-bis(2-chloroethyl)-N-nitrosourea, and nitrogen mustard. These cell lines may prove to provide a useful model system for evaluating the cellular and molecular processes which confer resistance or sensitivity in cancer treatment.