Two cell lines, T24 and MGH-U1, established from human urinary bladder carcinoma showed typically epithelial features. Although the proliferating cell population presented a polymorphic appearance including polygonal and elongated cells, ultrastructural studies failed to identify different types of cells in each cell line. Extensive cytoplasmic organelles, microvilli on the cell surface and cell continuity with the junctional complex were common findings in both the cell lines. Virus-like particle was not detected. Chromosome numbers of these cell lines were distributed from hypertriploidy to hypertext-raploidy. T24 cell line contained a large number of elongated cells and well-developed microvilli as compared with MGH-U1 cell line. The present results suggest that T24 cell line has been transformed to a relatively uniform cell population during the long-term culture as referred to in a previous report.