Keratin 8 and 18 are commonly used as tumorigenic markers for various types of carcinomas. They are known to be involved in cell migration, cell invasiveness, plasminogen activity and drug and radiation resistance. To ascertain a potential function for simple epithelium keratins in mammary adenocarcinoma in vivo, keratin-8-deficient mice (mK8) were mated with transgenic mice carrying the middle T oncogene driven by the MMTV promoter. The resulting mK8 knockout and control progeny carrying the middle T transgene developed mammary gland tumours with the same incidence. However, the onset of palpable mammary gland tumours occurred earlier in mK8 mutant than in control mice. This effect was prominent in males where the onset in control animals is delayed overall, because of the lower hormonal inducibility of the MMTV promoter. Metastatic foci were observed in the lungs of all females and of a few males, independently of the genotype. Histological analysis revealed no morphological differences of the tumorigenic cells in primary tumours nor in metastatic foci. As expected, keratin 8 was absent in the mK8 tumours. Keratin 7 (mK7), keratin 18 (mK18) and keratin 19 (mK19) protein were observed in both primary and metastatic foci. These results constitute the first in vivo analysis of the role of simple epithelium keratins in mammary carcinogenesis. It demonstrates that the latency, but not the incidence nor the morphological features, of PyV middle T-induced mammary gland tumours is affected by keratin 8 deficiency.