It is generally accepted that tumors are angiogenesis-dependent. For research and clinical purposes it would be very attractive to have a simple in vitro model that allows a rapid screening of the angiogenic potential of tumors and to study the effect of angiogenic inhibitors. In vitro angiogenesis models were developed, based on endothelial sprouting/tube formation on a collagen gel, using both tumor cell lines and tumor biopsies. Best results were obtained using conditioned medium of tumor cell lines. In this model it was found that the plasminogen fragment lysine binding site 1 (LBS-1) inhibited in vitro endothelial cell sprouting. This is the first demonstration that LBS-1, which includes angiostatin, is inhibitory for new vessel formation in an in vitro angiogenesis model. We conclude that the assay system allows for rapid and reliable screening of angiogenesis inhibitors.