Studies were made on the effect of various doses of SV40 on the transformation rate of bovine cells, and the transformed cells were studied with regard to the presence of infectious SV40, the presence of the specific complement-fixing (CF) “tumor” antigen, and the transplantability into the autologous host. It was shown that the time required for transformation varied between 42 and 407 days, being shorter with larger doses of virus. All the transformed cell lines contained SV40 in small amounts. The CF “tumor” antigen was found in the transformed cell lines except for early passages. Autotransplantation of subcutaneous tissue transformedin vitro gave rise in one calf to a small nodule, histologically defined as a spindle-cell sarcoma, and in two other calves a late inflammatory reaction was observed.