A new renal cell carcinoma (RCC) cell line (KRC/Y) has been established from a surgical specimen of a 41-yr-old Japanese female patient with RCC composed of both clear cells and granular cells. This cell line has been maintained for more than 15 mo. through 45 passages with a stable growth. KRC/Y cells have clear or eosinophilic polygonal cytoplasm and round to oval nuclei with one or two nucleoli, and proliferate in a pavementlike cell arrangement with a lack of contact inhibition. By electron microscopy, these cells contain abundant fat droplets and glycogen granules or well-developed organelles or both, which were also observed in the original tumor. The doubling time of these cells at the 15th passage was 73 h. The chromosome number was from 37 to 45 with a hypodiploid modal number of 42. Tumorigenicity was identified by tumor formation after subcutaneous injections of KRC/Y cells in nude mice, which showed close resemblance to the original tumor by light and electron microscope observations.