This study examined the effect of stable transfection of latent transforming growth factor-beta1 (TGF-beta1) cDNA into a predominantly polygonal, 4 nitroquinoline N-oxide (4NQO)-induced rat oral keratinocyte cell line. Seven polygonal and five spindle clonal populations were isolated that overexpressed TGF-beta1 protein by approximately two- to four-fold compared to vector-only transfected controls. Neutralisation experiments indicated that the majority of protein was in the latent form. There was no change in the proportion of polygonal and spindle cells in vitro after transfection with TGF-beta1 cDNA. Polygonal and spindle cells that overexpressed TGF-beta1 produced similar amounts of protein and grew more slowly in vitro than controls. The parent cell line and all transfected cells were growth inhibited (60-75%) by exogenous TGF-beta1. Orthotopic transplantation of the parent and the vector-only control cell lines resulted in primary tumours in the floor of the mouth in almost 100% (20/21) of athymic mice, with no evidence of bone resorption at the site of the primary tumour and pulmonary metastatic tumour deposits in some 40% (7/20) of these animals. The polygonal and spindle cells that overexpressed TGF-beta1 behaved similarly following orthotopic transplantation. A 96% (23/24) primary tumour take was evident following transplantation of cells that overexpressed TGF-beta1, with a significantly (P<0.02) higher number of animals showing bone resorption at the site of the primary tumour (35%; 8/23) compared to controls. By contrast, there was a significant (P<0.03) decrease in the number of animals with pulmonary metastases (4%; 1/23) following transplantation of TGF-beta1 overexpressing cells compared to controls. Overexpression of TGF-beta1 did not alter tumour cell differentiation in vivo. The results demonstrate that endogenous TGF-beta1 functions as a tumour suppressor in the rat-4NQO model of oral carcinogenesis without altering tumour cell morphology or differentiation but can also act to promote local bone resorption.