Retroviral expression of transforming growth factor-alpha does not transform fibroblasts or keratinocytes. Transforming growth factor alpha (TGF alpha) is a peptide so named because it helps to impart anchorage-independent growth to normal rat kidney (NRK) cells in vitro and is secreted by many rodent and human tumor cells. To directly investigate the transforming properties of this factor, we constructed a replication-defective murine retrovirus that expresses the human sequence coding for TGF alpha. Infection of NIH/3T3 cells with the TGF alpha retrovirus led to the integration of a transcriptionally active provirus and overexpression of biologically active TGF alpha, but failed to induce morphologic transformation. Similarly, the TGF alpha retrovirus failed to induce morphologic transformation of five other types of rodent fibroblasts. We also investigated the effect of TGF alpha expression on the growth of BALB/MK mouse keratinocytes, which require epidermal growth factor (EGF) for proliferation. We show that exogenously added TGF alpha is an extremely potent mitogen for BALB/MK cells. However, retroviral expression of TGF alpha in BALB/MK cells failed to relieve dependence on exogenously added EGF (or TGF alpha) for cell growth. These results suggest that overexpression of TGF alpha does not, by itself, transform rodent fibroblasts or keratinocytes. 