UV radiation of different wavelengths has been shown to alter proteins and induce nuclear factors in human cells in vitro. The in vivo effects on human skin and particularly those due to UVA are much less known. We therefore investigated in vivo the induction fo DNA damage and stress proteins by UVA I (340-450 nm, with small amount of visible) and UVA i + II (320-400 nm). Previously unexposed skin of 5 volunteers was irradiated with similar to 1 and 2 MED of UVA I and UVA I + II radiations, 4 mm biopsies were taken at 24 h post-UV and fresh frozen and formalin fixed, paraffin embedded 5 mu m sections were prepared. Immunohistochemistry was performed with antibodies to ferritin, p53, NF kappa B and pyrimidine dimers. UVA I induced a dose-dependent marked expression of ferritin in suprabasal epidermal and interstitial dermal cells, which was not obtained by UVA I + II. Induction of p53 induction also varied occording to the UVA spectrum used: UVA I + II was more efficient and produced staining throughout the epidermis, whereas UVA I only induced detectable effects in basal cell of the epidermis. Pyrimidine dimer induction was marked following both UVA I and UVA I + II radiations: following UVA I, it could be detected well into the dermis. NF kappa B was induced in a dose-dependent manner following both radiations in epidermal keratinocytes. Our experiments have demonstrated that 1 to 2 MED of UVA radiation induce in vivo in human skin easily detectable alterations depending on the UVA spectrum used