The bovine papillomaviruses are capable of transforming certain mouse fibroblast lines as well as certain rat fibroblast lines. The viral DNA in these transformed lines is maintained exclusively as extrachromosomal plasmids. The extrachromosomal nature of the viral DNA in these lines together with the selected malignant phenotypes has been utilized to develop the papillomaviruses as eukaryotic cloning vectors. We have shown that the complete genome cloned into a deletion derivative of pBR322 (pML2) is capable of serving as a shuttle vector between bacteria and eukaryotic cells. Eukaryotic and prokaryotic genes can be expressed in mammalian cells as part of BPV plasmids. We have shown that the human beta interferon gene can be inducibly regulated when maintained in a plasmid state in mouse cells. Using a stable plasmid containing the neomycin resistance gene (a phosphotransferase) from Tn5, we have the tissue range and host range of papillomavirus plasmid replication. This plasmid remains extrachromosomal in CV-1 cells (a monkey kidney cell line) and the cells do not exhibit any of the characteristics of transformed cells. When selected for drug resistance, specialized mouse cells including mouse epidermal cells, mouse hepatocytes, and mouse lymphocytes contain the DNA integrated.