The simian virus 40 (SV40) small t (t) antigen is known to be able to induce cell proliferation and to enhance the transforming activity of SV40 large T antigen. Here we report that t could also enhance the transforming activity of v-src oncogene. When t was transfected into the v-src-transformed NIH3T3 cells, the t-expressing stable clones grew faster and grew to higher density than did the parental or vector-transfected cells. Furthermore, these t-expressing cells also showed better plating efficiency and grew more efficiently in soft agar than did the parental or vector-transfected cells. More importantly, the t-expressing cells displayed high tendency to aggregate and detached easily from the dishes, while the parental or vector-transfected cells never exhibited such phenotype. This last observation suggests that t may affect the expression of adhesion molecules in the v-src-transformed NIH3T3 cells. Taken together, we concluded that t could enhance the transforming activity of v-src and alter the transformed morphology of v-src-transformed NIH3T3 cells.