To characterize the activity of the polyomavirus regulatory region, two hybrid marker genes were constructed. In the first construct, the early promoter regulates expression of the CAT gene and the late promoter regulates expression of the lacZ gene. In the second construct, the lacZ gene was placed under the control of the early promoter. The fusion constructs were introduced into the mouse germline. Gene expression was analysed in the generated transgenic mice. A pronounced cell-type specific activation of the transcriptional control region was found in different tissues of the developing embryo and in the adult animal. The control region is recognized and activated in early preimplantation embryos. Around the time of implantation, sequential activation of the Py regulatory region was first observed in differentiating cells. Stage- and tissue-specific expression were noted later in embryonic development. Comparing reporter gene expression on the single-cell level, the different viral promoters display identical expression patterns throughout ontogenesis. Quantitative analysis revealed that marker gene expression from the late promoter was significantly higher than from the early promoter. Furthermore, the cell-type specificity of the control region is not altered in the presence of its regulatory protein, the LT.