Deletion of 9p21 has frequently been observed in human bladder carcinomas. A candidate target suppressor gene, p16, was recently identified within this deleted region. In this study, we therefore investigated the loss of heterozygosity (LOH) of the p16 gene which is located on mouse chromosome 4, as well as its expression in mouse bladder carcinomas. We also studied the effects of normal cell contamination on LOH analysis using xenografts in CD-1(ICR) nude mice from B6C3F1 bladder carcinomas. We could not detect any LOH at the p16 locus in the mouse primary bladder carcinomas and xenografts. Surprisingly, overexpression of p16 was found in all primary mouse bladder carcinomas. Using microsatellite polymorphisms, a distinction could be made between PCR products derived from B6C3F1 and CD-1(ICR) nude mice. It was thereby confirmed that effects of normal cell contamination on LOH analysis are negligible when only tumor tissue is carefully sampled. The results suggest that abnormalities of p16 expression may be involved in mouse bladder carcinogenesis, but that gene deletion is not involved.