The Caki-2 cell line is one of a multitude of cell lines isolated in 1977 by Fogh et al. The cells are tumorigenic in nude mice. Recent evaluation (K. Pulkkanen and J. Parkinen, personal communication) of nude mouse tumors formed by this line in orthotopic and s.c. implantations were consistent with cystic papillary renal cell carcinoma according to the criteria of Kovacs et al. Ultrastructural features reported include microvilli and microfilaments with few mitochondria, lysosomes or lipid droplets. Frequent multilamellar bodies were observed, but no virus particles were seen.