Ependymomas were produced in 44 of 50 Syrian golden hamsters and in 9 of 31 outbred Swiss mice inoculated intracerebrally with high-titer, purified BK virus (BKV). Tumors contained a T-antigen that reacted with BKV-specific T-antibody in immunofluorescence and complement-fixation tests. A proportion of tumor-bearing animals had antibodies to BKV T-antigen in their sera. BKV could be rescued from two tumor cell lines by Sendal virus-mediated fusion with Vero cells. A low, or lack of, oncogenic activity was displayed by BKV inoculated sc, ip, or iv.