Since Hayflick's pioneering work in the early sixties, human diploid fibroblasts have become a widely accepted in vitro model system. Recently, Bayreuther and co-workers extended this experimental approach showing that fibroblasts in culture resemble, in their design, the hemopoietic stem-cell differentiation system. They found that the chemical agent mitomycin C accelerates the differentiation pathway from mitotic to postmitotic fibroblasts. We measured the response of endogenous glutathione levels after UVA irradiation (320-400 nm) in mitotic and mitomycin C-induced postmitotic human skin fibroblasts and foreskin-derived keratinocytes. The initial levels in mitotic foreskin derived human fibroblasts were 14.4 nmol glutathione per mg protein, whereas a 30% higher value was obtained in matching foreskin-derived keratinocytes. Similar elevated levels of this important intracellular free radical scavenging system were found in fibroblasts of a donor suffering from xeroderma pigmentosum. Furthermore, three to four times higher levels of glutathione in mitomycin C-treated mitotic fibroblasts have been determined. In mitotic skin fibroblasts, UVA irradiation resulted in a depletion of glutathione up to 90% following a fluence of 1.0 MJ/m2 UVA radiation. Higher initial glutathione levels were found in keratinocytes and mitomycin C-treated skin fibroblasts. In these fibroblasts a 70% depletion was detected and a much lower depletion (10-20%) was seen in some keratinocyte cell lines following fluences up to 1.0 MJ/m2. The depletion in skin fibroblasts was retained after 24 h following a fluence of 0.75 MJ/m2 UVA light. In view of the fact that glutathione has been shown to be involved in a variety of metabolic processes and plays a role in cellular protection against UVA radiation, our results imply that the fibroblast differentiation system is a very useful tool to unravel the complex mechanism of UVA-induced oxidative stress