Primary Chinese hamster embryo cell cultures generally yield cell lines with a finite lifetime in culture. However, these cells can be converted to continuous lines by exogenous 1-methylguanine and 7-methylguanine, two normal degradation products of tRNA. Continuous lines have also been obtained by treatment with 3-deazaguanine and the carcinogenic hydrocarbon 3,4-benzpyrene, but not with guanine, 3-methylguanine, or the carcinogen l-ethionine. Continuous lines do arise spontaneously at a low frequency, and lines derived after exposure to modified purines have been analyzed and compared to a spontaneously arising continuous line. The spontaneous Chinese hamster cell line has many characteristics of normal finite lines. These control cells exhibit a diploid karyotype, are not tumorigenic in nude mice, and have normal tRNA methyltransferase, methionine adenosyltransferase, and S-adenosylhomocysteine hydrolase activities. However, as compared to finite Chinese hamster cells, these cells are resistant to infection by the tumor virus SV40. The 1-methylguanine and 7-methylguanine transformed lines which have been analyzed in most detail differ greatly with regard to cell morphology. The former appear to be fibroblasts, while the latter are epithelial-like. The 1-methylguanine treated line exhibits a hyperdiploid karyotype with a modal chromosome number of 23. These cells are tumorigenic in nude mice forming well encapsulated, undifferentiated fibrosarcomas. The 7-methylguanine treated line is hypodiploid with a modal chromosome number of 21. This line is not tumorigenic in nude mice. The transformed cell lines derived by treatment with any of the modified purines or the carcinogen 3,4-benzpyrene all demonstrate elevated tRNA methyltransferase activity as well as elevated S-adenosylhomocysteine hydrolase activity. In addition, methionine adenosyltransferase activity is increased in the 3-deazaguanine and benzpyrene derived lines. Elevated levels of methylated degradation products of tRNA may then aid in maintaining the transformed state.