Patch tests with nickel on sensitive subjects induce a characteristic allergic reaction involving epidermal and dermal cells, as well as modulation of cytokines and adhesion molecule production. In order to gain further insight into the role of keratinocytes in this phenomenon, we assessed their activation state induced by Ni2+ by studying interleukin 1 (IL-1) production and intercellular adhesion molecule 1 (ICAM-1) expression, using normal human keratinocytes cultured in defined medium. In comparison with controls, the addition of subtoxic NiSO4 concentrations (0.1-20 micrograms/ml) to keratinocyte cultures induced a significant, but low release of IL-1 alpha and IL-1 beta at 24 and 48 h, detectable by enzyme-linked immunosorbent assay of the supernatants of treated cells. Moreover, IL-1 receptor antagonist was significantly increased in the supernatants and the cell extracts. Using fluorescence-activated cell sorter analysis, ICAM-1 expression at 24 h was found to be induced in a dose-dependent manner, reaching a level comparable with that obtained upon interferon-gamma (10 IU/ml) stimulation. Overall, these data confirm the existence of direct interactions between Ni2+ and keratinocytes, which generate immunological signals of major importance in the pathophysiology of allergic contact dermatitis.