Actual acidity of the skin surface as well as the capacity to neutralize topically applied 0,01 n NaOH were measured by means of a potentiometric procedure. 50 children without diseases of their skin showed pH values between 4,5 and 6,1 (mean pH = 5,24) when tested at the volar forearms. A significant shift towards higher acidity became apparent by pH readings (4,1--5,7; mean 4,73) in 25 patients suffering from mucoviscidosis. In both groups no difference could be found in regeneration of the skin acidity after exposure to NaOH. Our results favour the assumption that elevated skin surface acidity in patients with mucoviscidosis is due to qualitative or quantitative differences in the biochemical composition of the epidermis.