BK virus (BKV), a human papovavirus, was inoculated iv into 3-week-old Syrian golden hamsters. Between 2 1/2 and 9 months after inoculation, 82% of the animals developed tumors. The induced neoplasms were ependymoma, carcinoma of the pancreatic islets, osteosarcoma, adenocarcinoma, angiosarcoma, angioma, lymphoma, and seminoma. Hypersecretion of insulin, glucagon, C-peptide, and calcitonin was detected in tumors of pancreatic islets. BKV etiology of tumors was supported by the following evidence: 1) No tumors with BKV-specific markers appeared in animals given injections of buffer, animals inoculated with BKV neutralized by anti-BKV-specific serum, or uninoculated controls; 2) BKV tumor (T) antigen was detected by immunofluorescence and complement fixation tests in tumors of animals inoculated with infectious BKV and in transplanted tumors; 3) antibodies to BKV T-antigen were detected in sera of animals bearing primary or transplanted tumors; 4) BKV could be activated by Sendai virus-mediated fusion of neoplastic cells with susceptible Vero cells; and 5) no endogenous hamster oncornaviruses were found in tumors.