An adult renal carcinosarcoma was studied by light and transmission electron microscopy. A portion of the tumor was transplanted and serially passed in athymic mice. The neoplasm consisted in part of typical renal cell carcinoma. The sarcomatous portion showed definite differentiation into bone, cartilage, and skeletal and smooth muscle. Hilar lymph nodes were involved by carcinoma, whereas the renal vein and recurrent tumor in the flank incision consisted of sarcoma. Only the sarcomatous tissue was transplanted into mice, and no carcinoma appeared during serial passages.