The progression of normal Syrian hamster embryo cells to a neoplastic phenotype after treatment with a chemical carcinogen and continuous exposure to phorbol ester tumor promoters was studied in cell culture. Tumor promoters were able to rescue cell lines derived from individual carcinogen-treated colonies from a program of cellular senescence. In general, these cell lines did not grow in soft-agar medium or produce tumors in newborn hamsters at early passages but acquired these properties at later passages. These cell lines were used to study the temporal acquisition of a phenotypic characteristic of neoplastic rather than normal hamster embryo cells: the synergistic induction of the enzyme ornithine decarboxylase (ODC) by tumor promoter and serum growth factors (O'Brien, T. G., Saladik, D., and Diamond, L. Regulation of polyamine biosynthesis in normal and transformed hamster cells in culture. Biochim. Biophys. Acta, 632: 270, 1980). All cell lines that acquired neoplastic status with passage in culture exhibited the synergistic induction of ODC prior to their acquisition of the ability to either grow in soft-agar medium or produce tumors in newborn hamsters. Cell lines that responded to promoters with the synergistic induction of ODC accumulated greater levels of polyamines, especially putrescine, after promoter treatment than did normal cells. Tumor promoters did not affect the percentage of cells labeled with [3H]thymidine in preneoplastic cell lines, a finding similar to previous results in normal and neoplastic hamster cells. These studies demonstrate that tumor promoters can affect the early stages of neoplastic progression in carcinogen-treated Syrian hamster embryo cells and that a particular phenotypic property found in neoplastic hamster cells, the synergistic induction of ODC by tumor promoters and serum growth factors, is acquired by cell lines before they acquire neoplastic potential.