Loss of heterozygosity (LOH) is one of the most common genetic abnormalities in cancer. To define the role of LOH and chromosomal abnormalities at various stages of mouse mammary cancer progression, we analyzed the allelotypes and karyotypes of primary mammary tumors induced in CD2F, mice by two basic protocols, the classical multiple-dose 7,12-dimethylbenz[a]anthracene (DMBA) protocol and a novel protocol of combined medroxyprogesterone acetate (MPA) and DMBA. The advantage of the latter protocol is that its latency for tumor development is much shorter and its tumor incidence is higher than those of DMBA alone. To study more advanced stages of mammary tumor progression, we also analyzed mouse mammary tumors that had acquired autonomous growth and were transplantable into syngeneic hosts. The allelotypic studies were performed by means of microsatellite length polymorphism analysis with a minimum of two simple-sequence repeat markers per chromosome. We observed that MPA-DMBA-induced mammary adenocarcinomas, which in general arose earlier because of the growth promotion exerted by MPA, did not show any significant LOH and were essentially diploid. Tumors induced by DMBA alone, which on average took longer to develop, showed a higher frequency of allelic losses. LOH on chromosome 11 was observed in 30% of the cases. Chromosomes 4 and 8 were affected in 25% and 20% of the tumors, respectively. Interestingly, advanced stages of mammary tumor progression, represented by transplantable mammary tumors, showed a much higher level of genomic instability, specifically a very high frequency (66%) of LOH on chromosome 4. These findings indicate that chromosome 4 harbors a gene whose inactivation may play a role in the acquisition of more aggressive characteristics such as autonomous growth and transplantation ability.