The mitogenic effect of a new growth factor that we recently isolated from the acellular ascitic fluid of the Ehrlich ascites carcinoma grown in vivo was examined. We have called this factor EACF (Ehrlich ascites carcinoma factor). EACF caused initiation of DNA synthesis in the liver, submandibular gland, exorbital lacrimal gland and epithelium of the tongue of adult mice after i.p. injection at a protein concentration of 3 pg per 25 g of body weight. For all tissues examined, except the tongue, EACF initiated DNA synthesis at about 48 to 60 h after injection, with the maximum effect at approx. 85 h, and the stimulatory effect lasting approx. 60 h. The initiation of DNA synthesis in liver, which is normally characterized by only an occasional cell passing through the S phase by EACF is of particular interest. The initiation of DNA synthesis in the liver was not prevented by hypophysectomy. Evidence also indicates that a similar heat-labile growth promoting factor(s) is present in calf serum.

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