Abstract
Tissues from four cases of Kaposi's sarcoma developing in renal transplant recipients were studied by light and electron microscopic examination and by immunoperoxidase staining for Factor-VIII-related antigen. Ultrastructurally, the tumors in all four cases contained a variable mixture of cells, including endothelial cells, pericytes, fibroblasts, and myofibroblasts. These findings support the origin of Kaposi's sarcoma from primitive vasoformative mesenchyme. Immunoperoxidase staining for Factor-VIII-related antigen was limited to endothelial cells. In one case intracytoplasmic virus-like tubular complexes were seen. The significance of this finding is briefly discussed.