Integration and expression of human adenovirus type 12 (Ad12) were studied on two Ad12-induced tumor lines of C3Hf/OK mouse origin (IC and D tumors). Southern hybridization using whole Ad12 genome as probe demonstrated that DNA of IC and D tumors contained about 9 and 23 copies, respectively, of the entire Ad12 genome eqivalent per diploid cell. Left ends of Ad12 DNA appeared as off-size bands, indicating their linkage to different cellular DNAs. Southern hybridization using and Ad12 EcoRI-C fragment containing the E1 region as a probe revealed that intergration sites were much fewer than presumen numbers of integrated Ad12 genones. These results suggested that the viral and cellular DNA complex was repeated at integration sites. Northen hybridization using an Ad12 EcoRI-C fragment as a probe showed that tha E1 region was transcribed in both IC and D tumors. Much more intensive expression in the D tumor suggested possible dependence of the expression of the E1 region on the copy numbers of Ad12 genomes.