Murine sarcoma virus (MuSV)-transformed mouse fibroblasts produce polypeptide growth factors that stimulate cell division and colony formation in soft agar. These factors have molecular weights of approximately 25,000, 12,000, and 7,000, and are heat-stable, trypsin-sensitive, and capable of competing with epidermal growth factor (EGF) for membrane receptors. They differ from mouse EGF in molecular weight, inability to react with anti-EGF antibodies, and ability to induce anchorage-independent growth. These factors, termed sarcoma growth factors (SGFs), are a new class of polypeptide tropic factors that confer transformed-like properties on fibroblasts in vitro. SGFs were partially purified from the supernatant of MuSV-transformed 3T3 cells and showed EGF-competing activity and growth-stimulating activity. They stimulated fibroblasts to divide, overgrow in monolayer cultures, and form colonies in soft agar. These effects were observed within days of treatment and persisted as long as the peptides were present. SGFs did not cause permanent genetic changes in fibroblasts, as cells could cycle between transformed and untransformed states without genetic change. SGFs were tested for their ability to stimulate thymidine incorporation and soft agar colony formation. They showed dose-dependent colony growth and were stable to boiling but sensitive to trypsin and dithiothreitol. The 12,000 molecular weight protein was the most active in stimulating cell division in soft agar. SGFs were compared to EGF and found to compete for EGF receptors but were not immunologically related. They stimulated thymidine incorporation and soft agar colony formation, but EGF did not stimulate soft agar growth. SGFs were distinct from EGF in their ability to induce anchorage-independent growth. The study supports the hypothesis that MuSV transformation involves the production of polypeptide growth hormones. SGFs, produced by transformed fibroblasts, act on untransformed fibroblasts, suggesting they are direct effectors of cell transformation. Further studies are needed to determine if SGFs are direct products of the sarcoma virus or growth factors produced by the cellular genome in response to MuSV transformation.Murine sarcoma virus (MuSV)-transformed mouse fibroblasts produce polypeptide growth factors that stimulate cell division and colony formation in soft agar. These factors have molecular weights of approximately 25,000, 12,000, and 7,000, and are heat-stable, trypsin-sensitive, and capable of competing with epidermal growth factor (EGF) for membrane receptors. They differ from mouse EGF in molecular weight, inability to react with anti-EGF antibodies, and ability to induce anchorage-independent growth. These factors, termed sarcoma growth factors (SGFs), are a new class of polypeptide tropic factors that confer transformed-like properties on fibroblasts in vitro. SGFs were partially purified from the supernatant of MuSV-transformed 3T3 cells and showed EGF-competing activity and growth-stimulating activity. They stimulated fibroblasts to divide, overgrow in monolayer cultures, and form colonies in soft agar. These effects were observed within days of treatment and persisted as long as the peptides were present. SGFs did not cause permanent genetic changes in fibroblasts, as cells could cycle between transformed and untransformed states without genetic change. SGFs were tested for their ability to stimulate thymidine incorporation and soft agar colony formation. They showed dose-dependent colony growth and were stable to boiling but sensitive to trypsin and dithiothreitol. The 12,000 molecular weight protein was the most active in stimulating cell division in soft agar. SGFs were compared to EGF and found to compete for EGF receptors but were not immunologically related. They stimulated thymidine incorporation and soft agar colony formation, but EGF did not stimulate soft agar growth. SGFs were distinct from EGF in their ability to induce anchorage-independent growth. The study supports the hypothesis that MuSV transformation involves the production of polypeptide growth hormones. SGFs, produced by transformed fibroblasts, act on untransformed fibroblasts, suggesting they are direct effectors of cell transformation. Further studies are needed to determine if SGFs are direct products of the sarcoma virus or growth factors produced by the cellular genome in response to MuSV transformation.