We examined the effect of the stable transfection of latent TGF-beta 1 cDNA, under the control of a cytomegalovirus promoter in the expression vector pcDNA3, into a 4NQO-induced clonal rat oral keratinocyte cell line that formed undifferentiated spindle cell tumours following subcutaneous transplantation to athymic mice. Test cells containing latent TGF-beta 1 cDNA produced a 2.3-fold increase in TGF-beta 1 protein compared to pcDNA3 controls as demonstrated by ELISA. Neutralisation experiments indicated that the majority of the protein was in the latent form. Untransfected and transfected (containing either TGF-beta 1 cDNA or pcDNA3) cell lines were keratin negative and vimentin positive. Cells transfected with TGF-beta 1 were inhibited more than pcDNA3 controls when cultured in an anchorage dependent or independent environment. Subcutaneous transplantation of cells overproducing TGF-beta 1 resulted in tumours of significantly smaller volume than vector-only controls. Further, orthotopic transplantation of cells containing TGF-beta 1 cDNA to the floor of the mouth in athymic mice markedly inhibited the development of pulmonary metastases compared to vector-only controls. Both test and control cell lines in athymic mice formed undifferentiated tumours with a complete absence of keratin elaboration. Subcutaneous xenografts were recultured and cells containing the TGF-beta 1 cDNA produced a similar amount of TGF-beta 1 peptide as the cells containing pcDNA3 only. The production of TGF-beta 1 by both of the xenograft-derived cell lines was significantly less than the parent, pre-transplanted cell lines and the untransfected cell line. All of the cell lines were inhibited by exogenous TGF-beta 1. Our results demonstrate that autocrine TGF-beta 1 functions as a tumour suppressor in vitro and in vivo in 4NQO-induced spindle tumour cells that are growth inhibited by the ligand. Furthermore, tumour formation in athymic mice is associated with selection for a cell phenotype with diminished autocrine TGF-beta 1 production.