Fibroblastoid cell cultures derived from leukaemic bone marrow were successfully infected with BVV. After 2 months of subcultivation the cultures showed the appearance of foci of altered cells, suggestive of malignant transformation. Such foci were absent in non-inoculated cultures. Both control and inoculated cultures had a limited life span, i.e. neither of them could be developed into continuous transformed cell lines. The presence of at least some BVV genome functions in the inoculated cells was demonstrated (i) by immunofluorescence using a reference BVV serum, (ii) by detection in the supernatant culture fluid of sedimentable particles bearing RNA-dependent DNA polymerase activity with preference for Mg2+ ions, and (iii) by electron microscopic detection of scarce cell-associated virus particles in one of the infected cultures. Infectious BVV could not be rescued. In contrast to leukaemic bone marrow cultures, diploid human embryonic fibroblasts of various origin could not be infected with BVV.