Infection of secondary human embryonic kidney (HEK) cells with human papovavirus BK (BKV) resulted in cellular lysis and degeneration within 7 days. After 30 days, multilayered colonies of transformed cells were found and subcultured for analyses. These BK-HEK cells uniformly expressed the BKV T-antigen but were only 1% V-antigen positive. They produced infectious virus and were resistant to superinfection by BKV. They reached a saturation density of 1.3 x 10(5) cells per cm2 in medium with 5% fetal calf serum, were able to grow in medium containing 2% serum, and did not form colonies in soft agar or tumors in nude mice. Nonintegrated, superhelical BKV DNA was detected in the noncloned cells as expected because they were persistently infected and contained RNA transcripts complementary to both early and late regions of the BKV genome. Analysis of T-antigen-positive clonal isolates of these BK-HEK cells by the Southern technique revealed an absence of free viral DNA and the presencce of integrated BKV DNA sequences corresponding to the early region of the BKV genome. These studies demonstrate the stable transformation of human cells by BKV. However, the transformed human cells which retain and express part of the BKV genome do not fully manifest the growth properties of other papovarirus-transformed cells.