Malignant undifferentiated sarcomas were induced in 11 of 15 (73.3%) newborn Syrian hamsters by s.c. inoculation of a recombinant DNA (pBK/c-rasA) containing BK virus (BKV) early region gene and the activated human c-Harvey-ras(c-Ha-ras) oncogene derived from T24 bladder carcinoma. The two genes inoculated independently as well as a recombinant DNA of BKV early region gene and normal human c-Ha-ras proto-oncogene were not tumorigenic. Tumor-derived cell lines propagated in culture were immortalized and had growth characteristics consistent with a fully transformed phenotype. Tumors and tumor cell lines showed tandem insertions of pBK/c-rasA in high copy number and expressed BKV- and c-Ha-ras-specific transcripts as well as BKV T-antigen and c-Ha-ras protein with a molecular weight of 21,000. We conclude that BKV DNA requires interaction with other oncogenic functions for tumorigenicity. These findings may be relevant to the role of BKV in human neoplasia, where cooperation or synergism between BKV and cellular oncogenes could occur as an aspect of the multifactorial process of carcinogenesis.