We have shown previously that the putative prostate carcinoma cell lines TSU-Pr1 and JCA-1 share a common origin. The observation that these cell lines have p53 and Ha-ras mutations identical to those in bladder carcinoma cell line T24 prompted us to investigate their possible interrelations. We used cytogenetics and DNA profiling to compare the genetic backgrounds of the three cell lines. At least 12 structural chromosomal abnormalities are shared between T24, TSU-Pr1, and JCA-1 cells. DNA profiles were identical for all three cell lines. These results clearly indicate that the cell lines TSU-Pr1 and JCA-1 are not of prostatic origin but are derivatives of the bladder carcinoma cell line T24. TSU-Pr1 and, to a lesser extent, JCA-1 are frequently used as models in prostate cancer research, and numerous publications have appeared based on these lines. Several other T24 cross-contaminants have been identified in the past, and some of these, such as ECV304, continue to be used under the wrong identity. Our findings highlight the insidious problem that can occur when information regarding cross-contamination does not reach individual researchers and/or the importance of the problem is not fully acknowledged.