Abstract
Six cases of cystosarcoma phylloides were evaluated by ultrastructure and steroid receptor analysis. Electron microscopy of the lesions supported previous reports of a heterogeneous tumor consisting of pleomorphic mesenchyme and normal or proliferative epithelium. In each case estrogen and progesterone receptor analysis indicated the presence of a nonsaturable estrogen and progesterone 4S binding protein rather than a specific steroid receptor as suggested by previous studies.