During intercellular induction of apoptosis, transformed fibroblasts are eliminated through the action of neighbouring nontransformed cells. TGF-beta thereby plays three distinct and central roles. a) TGF-beta released by transformed cells or added exogenously to the assay system triggers nontransformed cells to release a shortlived apoptosis inducing factor, which is specifically directed against transformed cells. b) TGF-beta is involved in the maintenance of the transformed state, which is required for expression of sensitivity for intercellular induction of apoptosis. c) TGF-beta further sensitizes transformed cells through downmodulation of their endogenous survival factors, which control a constitutively expressed apoptosis machinery. These data demonstrate that TGF-beta which is utilized by transformed fibroblasts for the maintenance of their transformed state, causes recognition of transformed cells by their nontransformed neighbours and triggers and enhances an apoptosis-inducing response which finally causes elimination of potential tumor cells.