AbstractThe relationship between alkaline phosphatase activity and cell growth has been studied in hamster cells transformed by different carcinogens. About 90% of normal hamster embryo cells were constitutively positive for alkaline phosphatase activity (AP+). However, there were no AP+cells in cell lines transformed after treatment with the chemical carcinogens dimethylnitrosamine or 4‐nitro‐quinoline‐N‐oxide and 0.02% and 4% AP+cells in cell lines transformed by polyoma virus or Simian virus 40. The glucocorticoid hormone, prednisolone, induced alkaline phosphatase activity in 12% and 44% of the enzyme‐negative (AP−) cells in cell lines transformed by polyoma or Simian virus 40, but this hormone did not induce alkaline phosphatase activity in AP−cells from cell lines transformed after treatment with the chemical carcinogens. Treatment of polyoma transformed AP−cells with the mutagen N‐methyl‐N′‐nitro‐N‐nitro‐soguanidine produced AP+cells, whereas no AP+cells were found after mutagen treatment of AP−cells from the chemically transformed cell lines. Studies on spontaneous segregation in the polyoma transformed cell line has shown that AP+cells segregated AP−cells both in vitro and in vivo, although no spontaneous segregation was observed from AP−to AP+cells.AP+cells, compared to AP−cells, showed a decrease in DNA synthesis, cell multiplication, the ability to form colonies in soft agar and tumorogenicity in animals. AP−cells induced for alkaline phosphatase activity by prednisolone, showed the same growth properties in vitro as uninduced AP−cells. The decreased cell growth found in AP+cells which were constitutive for alkaline phosphatase activity was therefore not found in the hormone induced AP−cells. The results indicate that constitutive alkaline phosphatase activity appears to be related to the regulation of cell growth and that AP−c