Genetic analysis of radiation-induced lymphomas from p53 heterozygous or null mice has revealed a high frequency of genetic alterations on mouse chromosome 19. Detailed microsatellite analysis of chromosome 19 deletions identified three independent regions of loss of heterozygosity, one of which was refined to a 0.3 Mb interval that contained the Pten tumor suppressor gene. More than 50% of radiation-induced tumors from p53+/- and p53-/- mice showed heterozygous loss of one Pten allele. In most cases, the remaining allele was wild type and expressed, suggesting that Pten is a haploinsufficient tumor suppressor gene for mouse lymphoma development. This conclusion was supported by the detection of specific intragenic deletions in Pten in tumors that retained one wild-type allele. Pten heterozygous mice were just as sensitive as p53+/- mice to induction of tumors by radiation, and surprisingly, the double p53+/-Pten+/-mice were equivalent to p53 null mice in radiation sensitivity. Despite the fact that Pten appears to be a haploinsufficient tumor suppressor gene, most tumors from both the single and double heterozygous mice had lost the remaining wild-type allele. The mechanism of loss in all cases involved the complete chromosome, suggesting that it is driven by other tumor suppressor genes on this chromosome. This sensitized screen therefore identified complementary roles for Pten and p53 pathways in suppression of tumor development induced by radiation exposure.