In vitro methods are disclosed that rely on a novel phenomenon of secondary sprouting by cultured endothelial precursor cells, after angiogenic-like tube formation and collapse have occurred on a basement membrane matrix. Particularly disclosed is an in vitro method of isolating and expanding, from a mixed population of mammalian cells originating from a tissue sample, a cellular population enriched for endothelial sprout cells, including cells having one or more physiological and/or immunological features of endothelial precursor cells. Also disclosed is an in vitro method for a screening a substance for potential proangiogenic or antiangiogenic activity.