UV-A irradiation induces a time-dependent activation of AP-1 in NCTC 2544 human keratinocytes. 4 h after irradiation, a 2–3-fold increase in AP-1 activity is observed in human keratinocytes and fibroblasts. Activation is still detectable 24 h later. The UV-A induced AP-1 binding complex is shown to contain c-Fos and c-Jun proteins. Lipophilic vitamin E impedes UV-A induced lipid peroxidation but does not prevent AP-1 activation which is inhibited by N-acetylcysteine, a hydrophilic antioxidant. This finding suggests that UV-A-dependent AP-1 activation is sensitive to the cellular redox state but is not related to membrane lipid peroxidation.