The present disclosure provides a method of inhibiting angiogenesis within a tissue of interest by providing either intact or nicked .beta.2-Glycoprotein 1 (.beta.2GP1) to cells associated with the tissue. The presence of .beta.2GP1 inhibits angiogenesis within the tissue, in part by preventing neovascularization into the tissue. The disclosure also provides a method for treating tumors by providing .beta.2GP1 to the tumor.