To define the role of the alpha2beta1 integrin in pathologic angiogenesis, we investigated tumor-associated growth and angiogenesis in wild-type and alpha2-null mice. Our findings reveal that the alpha2beta1 integrin plays an important role in angiogenesis via regulation of VEGFR1 expression. When challenged with B16F10 melanoma cells, mice lacking alpha2beta1 integrin ex-pression exhibit increased tumor angiogenesis associated with up-regulated VEGFR1 expression. In contrast, there was no alpha2beta1 integrin-dependent difference in the angiogenic response to Lewis lung carcinoma (LLC) cells. Interestingly, whereas B16F10 cells secrete high levels of placental growth factor (PLGF), LLC cells produce high levels of VEGF, but low levels of PLGF. The alpha2beta1 integrin-dependent difference in angiogenesis was restored to LLC cells by expression of PLGF, strongly suggesting that the angiogenic phenotype and tumor growth in the alpha2-null host is dependent on specific interactions between the tumor cell and the genetically defined integrin repertoire of the host microenvironment. Thus integrin alpha2-null mice represent an example of genetic alterations of "the soil" determining response to the "seed."