In order to develop an in vitro predictive assay for the detection of contact sensitizers, we investigated the possible modulation of the expression of cell-surface molecules in the early phases of treatment of murine epidermal cells (EC) with known contact sensitizers. After in vitro treatment of Balb/c EC with the strong contact sensitizer, TNBS, Langerhans cells (LCs) demonstrated a rapid up-regulation of CD45, CD40, CD32/16 (Fc gamma RII/III) and CD23 (Fc epsilon RII) molecules. CD45 and CD40 were also rapidly up-regulated on the dendritic epidermal T cells. Interestingly, after treatment with this severe sensitizer, a marked induction of CD40 expression was found on a CD45 negative population, most probably keratinocytes. In contrast to these cell-surface molecules, I-Ad/I-Ed and CD90.2 expression were unchanged. No change was observed on the expression of CD45 and CD40 after treatment with a mild or a weak contact sensitizer, citral and citronellal respectively. In contrast, like TNBS, they up-regulated the expression of CD32/16 and CD23 on LCs. The irritant sodium dodecyl sulfate had no effect on all these cell-surface molecules. Our results indicated that in vitro, chemicals with allergic potential induced early specific phenotype changes that may represent an early-activated state of the cells. This state may be responsible for initiating the afferent phase of contact sensitivity in vivo. Based on these findings, it might be possible to develop an in vitro assay to reduce the number of experimental animals for a fast screening of contact sensitizers and for discriminating between mild contact sensitizers and irritants.