T147 is an SV40 mutant that makes a normal small t antigen and a large T antigen that is only 147 amino acids long. We have introduced a second mutation into the genome of T147 which eliminates its ability to encode small t antigen. We show that this double mutant is able to transform C3H10T1/2 mouse cells in a focus assay and F111 rat cells in an agar suspension assay, demonstrating that the transforming domain of T antigen is located within its amino-terminal 147 amino acids. We also show that the T147 mutant T antigen, like wild-type T antigen, has a nuclear location. However, in contrast to wild-type T antigen, which is also found in the plasma membranes of wild-type transformed cells, we fail to detect any mutant T antigen associated with the plasma membranes of T147 transformants.