Keratinocyte proliferation in normal human skin was induced by application of irritants: sodium lauryl sulfate (SLS) or non-anoic acid (NAA). Each irritant was applied at three different concentrations to normal human skin in vivo under occlusion for 24 h. The irritative response was visually assessed for erythema, and the number of cycling cells was calculated from the number of Ki67 positive cells in cryosections from skin biopsies taken at 0, 24, 48, 72 and 96 h. In addition, to determine the elemental content of the epidermal cells during the response, two epidermal strata were examined by X-ray microanalysis (XRMA). Both the application of SLS and that of NAA resulted in erythema, and the number of Ki67 positive cells was increased at 48 h. XRMA revealed an initial increase in the sodium/potassium (Na/K) ratio, indicating cell membrane injury. Due to an increasing potassium (K) concentration the Na/K ratio decreased after 24 h, which is compatible with proliferation, in accordance with the Ki67 data. This was more evident in SLS-stimulated skin, where also the concentrations of magnesium (Mg) and phosphorus (P) increased, an observation commonly made in proliferating cells. It appears that application of SLS results in a period of cell damage lasting about 24-48 h, followed by a period of proliferation. The effects of NAA appear more complicated, and proliferation may take place in parallel with cell damage.