Extracts of human neoplastic tissue (three Hodgkin's lymphomas; three gliomas and one liposarcoma) previously shown to induce angiogenesis in vivo did not stimulate the proliferation of bovine aortic endothelial cell monolayers when tested in vitro. Proliferation was induced by coculture of endothelial cells with human or animal macrophages or with supernatants derived from these cells. However, angiogenic extracts were chemotactic both for guinea pig peritoneal macrophages and human mononuclear cells in vitro. These results imply that tumour extracts act indirectly to induce angiogenesis in vivo via their effect on host macrophages.