New blood vessel formation, or angiogenesis, occurs through the migration of endothelial cells in elongated sprouts. These sprouts are directed preferentially towards the inciting stimulus. Several studies have demonstrated that certain chemical substances can stimulate angiogenesis. In these cases, endothelial cell migration towards the chemical stimulus may be due to a preferential migration of cells from lower to higher concentrations of the mediator. Such concentration gradient-dependent cellular migration has been termed chemotaxis. Using a modification of the Boyden chamber technique to measure chemotaxis in vitro, we have now found that extracts of various adult bovine tissues have potent chemotactic activity for vascular endothelial cells. Adult bovine serum lacks similar chemotactic activity.