Reduction of the intracellular glutathione concentration increased the sensitivity of transformed fibroblasts for intercellular induction of apoptosis by TGF-beta-treated nontransformed cells. This finding demonstrates a modulating role of glutathione for the induction of apoptosis in transformed cells and strengthens the involvement of reactive oxygen species in this process. Reduction of the intracellular glutathione concentration however, did not render nontransformed cells sensitive for intercellular induction of apoptosis or abrogate the resistance of BPV-transformed cells. Therefore, the intracellular glutathione concentration is not the key that determines sensitivity of transformed cells, insensitivity of nontransformed cells or resistance of BPV-transformed cells.