Abstract: Epidermal keratinocytes in culture have been shown to produce many cytokines, and their proteins have been identified in skin tissue samples. It has therefore been assumed that these cytokines are transcribed in vivo by the epidermis in response to contact allergens. In this report, in situ hybridization was used to detect the messenger RNAs for interleukin-1 alpha (IL-1 alpha), interleukin-1 beta (IL-1 beta) and tumour necrosis factor-alpha (TNF-alpha) in samples of human skin prior to and at various times after application of urushiol, the immunogenic component of poison ivy/oak. In sensitive subjects, IL-1 alpha and TNF-alpha mRNAs showed a progressive increase in transcript levels that paralleled the clinical and histological features of the inflammatory process. The time-course of the IL-1 beta response differed from that of IL-1 alpha and TNF-alpha, in that there was an early (by 6 h after urushiol administration) elevation in IL-1 beta mRNA that occurred before there was evidence of inflammation and had returned to background levels by 72 h when the reaction had reached its peak. In contrast to urushiol-sensitive subjects, urushiol-anergic individuals did not exhibit an increase in IL-1 alpha, IL-1 beta or TNF-alpha mRNA levels. The data provide evidence for an in vivo role for epidermal IL-1 alpha, IL-1 beta and TNF-alpha transcription in the regulation of IL-1 beta and TNF-alpha polypeptide levels in the epidermis in response to this common contact allergen.