Murine sarcoma virus (MuSV)-transformed mouse fibroblasts produce polypeptide growth factors that stimulate cell division in monolayer cultures and promote colony formation in soft agar. These factors, known as sarcoma growth factors (SGFs), exhibit three major peaks of activity with apparent molecular weights of 25,000, 12,000, and 7000. SGFs are heat-stable, trypsin-sensitive, and active in nanogram quantities. They compete for membrane epidermal growth factor (EGF) receptors but differ from EGF in molecular weight, lack EGF antibody reactivity, and enable anchorage-independent growth. The 12,000 molecular weight protein is the most active in stimulating cell division in soft agar. The SGFs do not produce permanent genetic changes in responding fibroblasts and can cycle cells between untransformed and transformed phenotypes. These findings suggest that SGFs are a new class of polypeptide tropic factors that confer the transformed phenotype on fibroblasts in vitro.Murine sarcoma virus (MuSV)-transformed mouse fibroblasts produce polypeptide growth factors that stimulate cell division in monolayer cultures and promote colony formation in soft agar. These factors, known as sarcoma growth factors (SGFs), exhibit three major peaks of activity with apparent molecular weights of 25,000, 12,000, and 7000. SGFs are heat-stable, trypsin-sensitive, and active in nanogram quantities. They compete for membrane epidermal growth factor (EGF) receptors but differ from EGF in molecular weight, lack EGF antibody reactivity, and enable anchorage-independent growth. The 12,000 molecular weight protein is the most active in stimulating cell division in soft agar. The SGFs do not produce permanent genetic changes in responding fibroblasts and can cycle cells between untransformed and transformed phenotypes. These findings suggest that SGFs are a new class of polypeptide tropic factors that confer the transformed phenotype on fibroblasts in vitro.