The analysis of the antigenic and transforming properties of the DB strain of the human polyomavirus BK Virus [BKV(DB)] is presented. Two genomes were molecularly cloned from a single virus preparation and were shown to represent viable virus; one clone [pBKV(DB)dl82] contained an 82 nucleotide deletion in the regulatory region relative to the second clone [pBKV(DB)]; pBKV(DB)dl82 demonstrated enhanced lytic and transforming activities relative to pBKV(DB). BKV(DB) is antigenically distinct from the prototype Gardner strain of BK Virus, and 50 to 60% of the population display serological evidence of BKV(DB) infection. Implications of the existence of antigenic variants on estimation of BK virus prevalence in the population are discussed.