The number of chromatid breaks in peripheral blood lymphocytes (PBL) after exposure to bleomycin in the S/G2 phase of the cell cycle (in the literature referred to as 'mutagen sensitivity') is associated with an increased risk of environmentally related cancers, including oral cancer. The aim of this study was to elucidate whether mutagen sensitivity measured in lymphocytes actually reflects chromosomal instability of normal cells in the areas in which tumors develop. Therefore, bleomycin-induced chromosomal damage in and growth inhibition of cultured oral fibroblasts and oral keratinocytes from 30 persons were compared with the standard mutagen sensitivity score in PBL. A correlation was found for the percentage of aberrant metaphases between PBL and oral fibroblasts but not for the number of breaks per cell. These data do not allow a conclusion to be drawn on the use of fibroblasts to study cancer risk. Within the fibroblasts it was found that a high number of breaks per cell was associated with less growth inhibition, indicative of damage-resistant growth. Oral keratinocytes were extremely sensitive to bleomycin, as indicated by a strong cell cycle block which resulted in a mitotic index too low to determine chromosomal breaks. Moreover, in the cell proliferation assay keratinocytes were found to be 100 times more sensitive as compared with fibroblasts. There was no correlation between bleomycin sensitivity of keratinocytes compared with fibroblasts from a single patient as measured by growth inhibition. This may be due to the strong influence of alcohol consumption by the subjects, which was found to increase the sensitivity of keratinocytes but not of PBL and fibroblasts. In conclusion, oral fibroblasts but not keratinocytes can be used to measure sensitivity for chromatid breaks. The apparent influence of environmental factors on keratinocytes makes them a useful source to study exposure characteristics but limits their application for the determination of genetic factors.