Human skin has an acid mantle of pH 4-6, contrasting with the almost neutral pH of the interior body and implying the existence of a pH gradient over the horny layer that might influence a variety of epidermal processes. In an attempt to characterize the pH gradient, we applied a glass electrode to the volar surface of the forearm before and after consecutive strippings with sello-tape. Before stripping, the surface pH (mean +/- SD) was 4.5 +/- 0.2 in men (n = 7) and 5.3 +/- 0.5 in women (n = 7), the values gradually increasing to pH 6.9 +/- 0.4 in men and 6.8 +/- 0.5 in women after about 100-120 tape strippings, which completely removed the stratum corneum. When plotted against the number of strippings, the pH values usually conformed to a sigmoid curve with inflection (50% change) after about 60 strippings, at a level corresponding histologically to the lower third of stratum corneum. Similar gradients were found also in skin of the abdomen and calf. Stripping with cyanoacrylate resin produced a similar gradient, even though this form of stripping was 10 times more effective. The healing process after tape stripping was studied by determining pH and transepidermal water loss in 5 persons over a period of 14 days. The importance of the re-established pH gradient is discussed in relation to the many pH-dependent enzymes operating in stratum corneum.