Endothelial cells derived from pig aorta were cultured in Petri dishes containing Sarcoma 180 cells growing in Millipore diffusion chambers. The presence of tumor cells consistently increased the colony-forming ability of the endothelial cells, an effect not produced by control-killed cells or 3T3 fibroblasts. This observation suggests that the cultured tumor cell retains the ability to produce diffusible substances that stimulate endothelial cell growth. These products may be similar to the angiogenic factors elaborated by malignant cells in vivo. The use of such a method may allow a closer study of the effect of tumor cells on the growth of a variety of tissues.