Purified tumour angiogenesis factor (TAF) obtained from rat Walker 256 carcinoma and found to induce neovascularization in vivo was examined for its effect on endothelial cell cultures of capillary (CBEC), cow aorta (CAEC) and pig aorta (PAEC) in vitro. Treatment with TAF increased the growth of capillary but not aortic endothelial cells, and then only when the cells were growing on a native collagen substratum. These data show an important growth difference between endothelial cells, in that the ability to proliferate in response to TAF depends not only on the substratum used but also on the vascular origin of the cells.