The formation of blood vessels in vivo (angiogenesis) is an important process and is usually initiated in response to injury, tumor growth, or normal tissue development. We have studied the effect of human interferon (IFN) alpha (α) and gamma (γ) on the capillary‐like network formation in an in vitro model of angiogenesis using human umbilical vein endothelial cells (HUVEC). When HUVEC cells are plated on Matrigel (reconstituted basement membrane matrix enriched in laminin), a network of capillary like structures (endotubes) rapidly forms. IFN‐α enhanced the tube formation in a dose‐dependent manner, whereas IFN‐γ significantly inhibited the tube formation. In addition, both the enhancement and inhibition of angiogenesis by IFN‐α and γ was found to be greater if the cells were pretreated with IFN for 12 hr before plating on the Matrigel. These results suggest that IFN may play an important role in several vascular processes including early stages of wound healing, recanalization of thrombi, tumor growth, metastasis, normal growth, and development.