AbstractWe have adapted T24P, a tumorigenic subline of the T24 human bladder cancer cell line, to grow in 5 mM butyrate. In the presence of butyrate, the adapted cells (T24P/B) grow more slowly than the unadopted cells (T24P/C), have a lower saturation density, increased serum requirement for growth, loss of ability to form colonies when plated at low cell density, and decreased ouabain sensitivity. Morphologically, T24P/B cells in butyrate are large and flattened with increased cytoplasm. When T24P/B cells are grown without butyrate, the morphological changes, growth rate, plating efficiency, and ouabain sensitivity return to those of T24P/C. While the saturation density increases, it does not return to levels of T24P/C, and the size of colonies never reaches that of the T24P/C colonies. Both T24PJC and T24P/B are tumorigenic in nude mice, however, the T24P/B tumors differ grossly and microscopically from those produced by T24P/C in that they contain large cystic structures filled with clear fluid and lined by transitional cell epithelium with flattened surface layers. Although the transformed phenotype and tumorigenicity of T24P are modified by adaptation to growth in butyrate, no significant changes in ras oncogene RNA or protein expression were identified.