As skin barrier modulating compounds, fatty acids are frequently used in formulations for transdermal or topical delivery. In this study the effects of oleic acid on keratinocytes in vitro was compared with its in vice skin irritancy in humans. Dose- and time-dependent effects of oleic acid were examined in submerged human keratinocyte cultures,in reconstructed human epidermis (RE-DED), and in excised human skin, using alterations in morphology and changes in interleukin-1alpha mRNA levels as endpoints. In vitro results were compared with responses of living human skin after topical application of oleic acid, using non-invasive bioengineering methods. Direct interaction of oleic acid and submerged keratinocyte cultures resulted in cell toxicity at very low concentrations of the fatty acid. By contrast, when oleic acid was applied topically on RE-DED or on excised skin, no alterations in morphology were observed. Modulation of stratum corneum thickness indicated a key role of the stratum corneum barrier in the control of oleic acid-induced toxicity. In agreement with these findings, no epidermal tissue damage was seen in vivo, whereas oleic acid induced a mild but clearly visible skin irritation and inflammatory cells were present in the upper dermal blood vessels. Small amounts of oleic acid induced IL-1alpha mRNA expression in submerged keratinocyte cultures, whereas in RE-DED and in excised skin, IL-1alpha mRNA levels were increased only when the concentration applied topically was at least. two orders of magnitude higher. It is concluded that minute amounts of oleic acid are sufficient to cause local (i.e. inside the viable epidermis) modulation of cytokine production. These concentrations do not affect morphology but induce skin irritation in vitro. To achieve comparable effects in the skin, much higher topical doses are needed than expected according to the locally required levels, owing to the rate-limiting transport of the fatty acid across the stratum corneum barrier.