A retroviral construct encoding polyoma middle-sized T antigen was used to generate transformed endothelial cell lines from heart (H5V), brain (B9V), and whole-embryo (E10V) of C57BL/6 mice. When injected into syngeneic recipients, H5V and the less studied B9V and E10V cells caused vascular tumors which, depending on the number of cells inoculated, regressed or progressed, leading to death of the host. When H5V cells were injected into immunodeficient mice, tumors were observed with inocula which did not form lesions in immunocompetent recipients and regression did not occur. Treatment with anti-LFA-1, anti-Thy-1.2, and anti-CD8 antibodies abolished rejection; anti-CD4 was a somewhat less effective inhibitor of resistance. Animals with progressive tumors exhibited secondary lesions in various organs with prominent skin involvement in nude mice. Histologically, the tumors had the appearance of a hemangioma, with areas resembling Kaposi sarcoma. Cells lining vascular lacunae had the morphological features of injected H5V cells. The lesions were characterized by prominent neovascularization and mononuclear cell infiltration. Southern blot hybridization analysis revealed that approximately 5% of the cells in the tumor mass were transplanted H5V cells. Thus, the H5V transformed endothelial line causes vascular lesions that are sustained to a large extent by recruitment of host cells and manifests full malignant behavior only in immunocompromised hosts. The hypothesis of a tumor sustained by a minute proportion of transformed cells, which recruit host elements and express full malignant behavior only in immunodeficient hosts, would account for several features of some vascular neoplasms in man.