Using an in vitro assay for CD80 promoter activity, the transcriptional response of primary cultured human keratinocytes from different donors in response to allergens and irritants was studied. The CD80 promoter activity was increased reproducibly after exposure to certain chemicals. The epithelial cell lines HeLa and HaCaT also increased CD80 transcriptional activity in response to exposure to a panel of different allergens and irritants. Normal or immortalized keratinocytes responded reproducibly with a unique pattern to a panel of chemicals in which the culture increases CD80 transcription after exposure to certain chemicals and has no increase in CD80 transcription after exposure to other chemicals. Some keratinocyte cultures responded positively to certain chemicals whereas others did not respond to the same chemicals. Cultured keratinocytes from 16 of 20 adult donors (80%) responded to nitrochlorobenzene in vitro by increasing CD80 transcriptional activity, thus predicting subsequent sensitization to this same hapten (chi2= 7.2, P <0.0073). Thus, a keratinocyte CD80 gene expression assay exhibits good sensitivity in predicting allergic contact dermatitis for sensitizers such as dinitrochlorobenzene. These data suggest that there is a rationale for developing the use of gene-transcription-based predictive assays to identify individual susceptibility to cutaneous reactivity after exposure to allergens and irritants.