Normal human fibroblasts (the OUMS-24 strain), derived from a 6-week-old human embryo, were transformed (into the OUMS-24F line) and immortalized by repeated treatments (59 times) with 4-nitroquinoline I -oxide (4NQO). Treatment began during primary culture and ended at the 5 I st population doubling level (PDL). At the 57th PDL (146 days after the last treatment), morphologically altered, epithelial-type cells appeared, began to grow and became immortal (now past the 100th PDL). However, the control fibroblasts, which were not treated with 4NQ0, senesced at the 62nd PDL. The finding that extensive, repeated treatments with 4NQ0 are required for the immortalization of normal human cells, indicates that multiple mutational events are involved in the immortalization of human cells in general. In other words, immortalization itself seems to be a multi-step process. Karyotypic analysis showed that many cells were hypodiploid before immortalization, but that afterwards chromosomes were distributed broadly in the diploid to tetraploid regions. The immortalized cells showed amplification and enhanced expression of c-myc. Two-dimensional electrophoretic analysis showed that the number of disappearing cellular proteins was greater than the number of the newly appearing ones after the cells became immortalized. Since the immortalized cells showed neither anchorageindependent growth nor tumorigenicity, they are useful for studying factors that can contribute to multi-step carcinogenesis in human cells. In addition, genetically matched normal (OUMS-24) and immortalized (OUMS-24F) cells will be useful for analyzing the genes related t o cellular mortality and immortalization.