We have previously reported that both 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF) can stimulate the synthesis rate of EGF receptors. We now show that the MDA468 breast cancer cells express the mRNA for the EGF-like molecule, transforming growth factor-α (TGF-α), and demonstrate that TPA or EGF cause an accumulation of both EGF receptor and TGF-α mRNA. The levels of EGF receptor mRNA paralleled our earlier protein data, with peak accumulations of 2-3-fold with 10-9 M EGF and 3-5-fold with 100 ng/ml TPA seen between 6 and 8 h. A 7-fold accumulation of TGF-α mRNA was seen following 4 h of treatment with TPA, and a 2-fold accumulation was seen after 8 h with EGF. These changes in EGF receptor and TGF-α mRNAs were observed in the absence of any change in the mRNA level of the α-subunit of hexosaminidase A (a lysosomal enzyme), demonstrating some degree of specificity. Detectable quantities of immunoreactive TGF-α accumulated in the cell culture medium of MDA468 cell treated with the blocking anti-EGF receptor monoclonal antibody B1D8 while no immunoreactive TGF-α was detected in the medium of cells with unblocked receptors. The concentration of B1D8 used was sufficient to block the binding of exogenously added 125I-EGF to undetectable levels but had only minor effects on cell growth and no effect on the expression of the TGF-α and EGF receptor mRNA.