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The alkaline phosphatase activity was studied in embryonic lung human fibroblasts grown in vitro. The total enzyme activity per flask was constant during the log-phase of culture growth to increase significantly after confluence. The increase in the activity did not depend on the rate of culture growth, and was seen to start after the mean density equal to (37.5 +/- 5.9) X 10(3) cells/cm2 flask surface. The stimulation of cell growth may presumably slow down the synthesis of alkaline phosphatase, which resumes after the contact inhibition of DNA replication.

A new strain of the embryonic human fibroblasts L-68 was obtained and thoroughly characterized. It completely met all the requirements of the International Committee on the Cells Cultures. This strain can be recommended as a substrate for production of viral vaccines, diagnostic preparations and for research purposes.

A study was made of the relationship between the activity of alkaline phosphatase and the proliferation of cultured human cells with different replicative potentials. It is shown that alkaline phosphatase plays a role as one of endogenic stimulators of cellular proliferation. The ageing of diploid cells is accompanied by a decrease in the enzyme activity. Maximum activity was observed during a period of logarithmic cell growth. Addition of placental alkaline phosphatase to the synchronized diploid cells stimulated DNA synthesis in the S-phase of the cell cycle. Heteroploid cells with a high growth rate possessed a 30-100 times higher alkaline phosphatase activity than in the diploid cells. Under certain conditions alkaline phosphatase may presumably function as a proteinkinase.