A growth-stimulating factor released by cultured mouse mammary tumor cells.

Medium from mouse mammary tumor cell cultures con tained a growth-stimulating factor. The assay for growthstimulating factor was performed on density-inhibited cul tures of mouse embryo cells. Addition of nondialyzed or dialyzed mouse mammary tumor cell culture medium caused an increase in DNA synthesis and mitosis in densityinhibited cells. Dialyzed medium was more active than non dialyzed medium in stimulating growth. Normal mouse mammary cell culture medium also showed similar growthstimulatory activity, but to a lesser extent than that showed by tumor cell culture medium. The growth-stimulating factor was precipitated from the culture medium with 60% saturated ammonium sulfate.