The closely related mammalian TGF-betas (TGF-beta 1, TGF-beta 2 and TGF-beta 3) are potent inhibitors of proliferation of many cell types in vitro. TGF-beta 1 has been demonstrated to be growth inhibitory in vivo for epithelial, endothelial, myeloid and lymphoid cells. Utilizing skin keratinocytes as a model system for studying the mechanism of TGF-beta 1-induced growth inhibition, it has been demonstrated that TGF-beta 1 rapidly inhibits transcription of the c-myc gene. Antisense c-myc oligonucleotides inhibit proliferation of keratinocytes as effectively as does TGF-beta 1, indicating that TGF-beta 1 suppression of c-myc expression is an important component of this growth inhibition. Studies utilizing DNA tumour virus transforming gene constructs have shown that the retinoblastoma gene product, pRb, or a related protein, is needed for TGF-beta 1 suppression of c-myc transcription. Thus, TGF-beta 1 may act through a tumour suppressor gene product, pRb, to suppress transcription of a proto-oncogene, c-myc, and subsequently inhibit cell proliferation.