A soluble factor, termed TAF, has been isolated from human and animal tumors that is mitogenic for capillary endothelial cells and responsible for tumor angiogenesis. This factor induces the growth of new capillaries and is essential for the vascularization of tumors. The study demonstrates that TAF can be isolated from various tumor types, including Walker 256 carcinoma, B-16 melanoma, neuroblastoma, Wilms tumor, and hepatoblastoma. The factor was purified through a series of steps involving centrifugation, filtration, and chromatography. The purified TAF showed strong neovascularization activity in the rat dorsal air sac assay, with the highest activity observed in fraction II. The factor was found to be sensitive to certain enzymes and heat but resistant to others. TAF contains approximately 25% RNA and 10% protein, with the protein moiety being essential for its mitogenic effect on endothelial cells. The study suggests that TAF is responsible for the final stage of tumor growth, where new capillaries are formed to supply nutrients to the tumor. The findings have potential therapeutic implications, as blocking TAF could arrest tumor growth at an early stage. The rat dorsal air sac assay is a reliable method for measuring neovascularization, although it is technically demanding. The study highlights the importance of understanding tumor angiogenesis for the development of effective cancer therapies.A soluble factor, termed TAF, has been isolated from human and animal tumors that is mitogenic for capillary endothelial cells and responsible for tumor angiogenesis. This factor induces the growth of new capillaries and is essential for the vascularization of tumors. The study demonstrates that TAF can be isolated from various tumor types, including Walker 256 carcinoma, B-16 melanoma, neuroblastoma, Wilms tumor, and hepatoblastoma. The factor was purified through a series of steps involving centrifugation, filtration, and chromatography. The purified TAF showed strong neovascularization activity in the rat dorsal air sac assay, with the highest activity observed in fraction II. The factor was found to be sensitive to certain enzymes and heat but resistant to others. TAF contains approximately 25% RNA and 10% protein, with the protein moiety being essential for its mitogenic effect on endothelial cells. The study suggests that TAF is responsible for the final stage of tumor growth, where new capillaries are formed to supply nutrients to the tumor. The findings have potential therapeutic implications, as blocking TAF could arrest tumor growth at an early stage. The rat dorsal air sac assay is a reliable method for measuring neovascularization, although it is technically demanding. The study highlights the importance of understanding tumor angiogenesis for the development of effective cancer therapies.