Short-term cultures of endothelial cells from fetal bovine heart and aorta consistently display increased growth rates when crude tumor cell homogenates from the Walker 256 adenocarcinoma (ascites and solid forms as well as tumor cells from a suspension culture) are added to the culture media. The tumor-derived material produces growth stimulation in both sparse and confluient endothelial cell cultures. Homogenates of embryonic tissues and cultured cells show similar growth-promoting effects; corresponding material from various adult tissues is ineffective. The nonendothelial cells that were tested generally show no growth response or only a slight positive growth response to the tumor cell homogenates. The results indicate the feasibility of using this in vitro system as a paradigm of the tumor-induced vascularization process.