Immortalization (senescence bypass) is a critical rate-limiting step in the malignant transformation of mammalian somatic cells. Human cells must breach at least two distinct senescence barriers to permit unfettered clonal evolution during cancer development: (1) stress- or oncogene-induced premature senescence (SIPS/OIS), mediated via the p16-Rb and/or ARF-p53-p21 tumour-suppressive pathways, and (2) replicative senescence triggered by telomere shortening. In contrast, because their telomerase is constitutively active, cells from small rodents possess only the SIPS/OIS barrier, and are therefore useful for studying SIPS/OIS bypass in isolation. Dermal fibroblasts from the Syrian hamster (SHD cells) are exceptionally resistant to spontaneous SIPS bypass, but it can be readily induced following exposure to a wide range of chemical and physical carcinogens. Here we show that a spectrum of carcinogen-specific mutational and epigenetic alterations involving the INK4A (p16), p53 and INK4B (p15) genes are associated with induced SIPS bypass. With ionizing radiation, immortalization is invariably accompanied by efficient biallelic deletion of the complete INK4/CDKN2 locus. In comparison, SHD cells immortalized by the powerful polycyclic hydrocarbon carcinogen benzo(a)pyrene display transversion point mutations in the DNA-binding domain of p53 coupled with INK4 alterations such as loss of expression of p15. Epimutational silencing of p16 is the primary event associated with immortalization by nickel, a human non-genotoxic carcinogen. As SIPS/OIS bypass is a prerequisite for the immortalization of normal diploid human epithelial cells, our results with the SHD model will provide a basis for delineating combinations of key molecular changes underpinning this important event in human carcinogenesis.