The human papovavirus, BK, appeared weakly oncogenic in newborn hamsters and was able to induce in vitro transformation of rat kidney cells. The infectivity of BK virus DNA was determined by employing the DEAE-dextran method. In human embryonic cells the infectivity was approx. 105 p.f.u./µg of DNA. The transforming ability of BK virus in primary rat kidney cells was measured by employing the calcium method and appeared to vary from 1 to 10 foci per µg of DNA.