Angiogenin is a 14 kD protein, initially isolated as a tumour-cell secreted product but subsequently found to be a normal constituent of human plasma. It is a potent inducer of blood vessel formation on the chorioallantoic membrane of the chick embryo. Chemical characterization of the protein reveals a remarkable homology to the pancreatic ribonuclease family and has led to the identification of a unique ribonucleolytic activity for angiogenin. It is a particularly potent inhibitor of in vitro protein synthesis. Treatment with placental ribonuclease inhibitor abolishes the biological and enzymatic activities of angiogenin, an effect with important mechanistic, physiological and pharmacologic implications.