The authors describe the isolation and characterization of a soluble factor from human and animal tumors that promotes angiogenesis, or the growth of new blood vessels. This factor, termed tumor-angiogenesis factor (TAF), is responsible for the formation of new capillaries in tumors. The study uses a rat dorsal air sac assay to measure neovascularization, which is induced by TAF. The TAF is extracted from various tumor types, including Walker 256 carcinoma, B-16 melanoma, human neuroblastoma, Wilms tumor, and hepatoblastoma. The biological activity of TAF is preserved under various conditions, such as digestion with ribonuclease, heating, and storage at different temperatures. The TAF is found to contain RNA, protein, and carbohydrate, with the protein moiety being essential for its mitogenic effect on capillary endothelial cells. The authors suggest that blocking TAF could potentially arrest solid tumors at a small size, similar to how bacterial colonies cease growth when nutrient diffusion becomes insufficient. The findings have potential therapeutic implications for understanding and treating tumor growth.The authors describe the isolation and characterization of a soluble factor from human and animal tumors that promotes angiogenesis, or the growth of new blood vessels. This factor, termed tumor-angiogenesis factor (TAF), is responsible for the formation of new capillaries in tumors. The study uses a rat dorsal air sac assay to measure neovascularization, which is induced by TAF. The TAF is extracted from various tumor types, including Walker 256 carcinoma, B-16 melanoma, human neuroblastoma, Wilms tumor, and hepatoblastoma. The biological activity of TAF is preserved under various conditions, such as digestion with ribonuclease, heating, and storage at different temperatures. The TAF is found to contain RNA, protein, and carbohydrate, with the protein moiety being essential for its mitogenic effect on capillary endothelial cells. The authors suggest that blocking TAF could potentially arrest solid tumors at a small size, similar to how bacterial colonies cease growth when nutrient diffusion becomes insufficient. The findings have potential therapeutic implications for understanding and treating tumor growth.