During the course of evaluating the interaction between allergens and keratinocytes in the pre-immunological phase of contact sensitization, we have studied the effects of paraphenylene diamine (pPD) on membrane lipid peroxidation and on intracellular antioxidant levels in cultured human keratinocytes. pPD is an aromatic amine which undergoes spontaneous oxidation in culture medium, generating short-lived free radical species including oxyradicals. Following exposure to non-toxic concentrations of pPD (0.5-10 micrograms/ml), we have evaluated the fatty acid pattern of membrane phospholipids as a target of peroxidative damage, and the intracellular level of reduced glutathione (GSH), the activity of superoxide dismutase (SOD), and that of catalase (CAT) as parameters of the antioxidant system. Depending on pPD concentration and the period of exposure, peroxidative damage with a significant decrease in membrane polyunsaturated fatty acids, was detected. Concentrations between 0.5 and 2 micrograms/ml produced an initial increase and then a decrease in both SOD and CAT activities, and in the oxidation of GSH, up to 12 h. After 24 h, when all the pPD had decomposed, recovery of the initial levels of the antioxidants was detected. Concentrations over 5 micrograms/ml induced a progressive decrease in both the enzymatic activities and the GSH concentrations. These results are consistent with the view that oxidative stress can be an essential event in the pre-immunological phase of contact sensitization.