Angiogenin is a potent inducer of blood-vessel formation with ribonucleolytic activity. Angiogenin binds to high affinity endothelial cell receptors and with lower affinity to extracellular matrix components. Here we report the effect of copper and zinc on these interactions. There was a 4.3-fold increase in angiogenin binding to calf pulmonary artery endothelial cells in the presence of Cu2+ in vitro. A 3.8-fold increase was observed with Zn2+, whereas Ni2+, Co2+, or Li+ had no effect. Specific angiogenin binding to the lower affinity matrix sites was increased by 2.7- and 1.9-fold in the presence of Cu2+ and Zn2+ respectively. Metal ion affinity chromatography and atomic absorption spectrometry were used to show the direct interaction of angiogenin with copper and zinc ions. Angiogenin bound 2.4 mol of copper per mole of protein. We suggest that copper, a modulator of angiogenesis in vivo, may be involved in the regulation of the biological activity of angiogenin.