Unlike cell lines transformed with Polyomaviruses, transformants derived by focus formation or colony formation in agarose medium after transfer into rat fibroblast cells (FR3T3 line) of Bovine Papilloma Type 1 (BPV1) DNA were consistently observed to grow poorly in suspension and to remain highly serum dependent for growth in culture. These cells did not produce detectable amounts of plasminogen activator, and kept the flat morphology and organized cytoskeleton characteristic of the normal fibroblast. However, they induced in syngeneic animals the development of tumors with a greater invasive potential than tumors induced by Polyomaviruses. By contrast with the original transformants, cells recovered from the tumors grew efficiently in suspension and produced high levels of plasminogen activator. They still had, however, extended cytoskeletal structures and remained completely dependent on high serum concentrations for growth in culture. The stepwise transformation process induced by BPV1 thus appears strikingly different from that previously observed with polyoma and SV40 viruses. The observed changes in transformation phenotype between transformed line and tumor cells do not correlate with any important modification of the number of autonomous copies of the viral genome, nor with any rearrangement of viral sequences detectable at the level of the blot analysis.