Recent epidemiological studies have brought to light a possible link between premalignant or neoplastic skin lesions (Bowen disease, squamous carcinoma) and occupational exposure to 4,4'-bipyridyl (4,4'B), a precursor in the synthesis of paraquat herbicide. The present study used a serum-free cell culture of normal human keratinocytes (NHK) and two skin-equivalent models to test the effects of exposure to different concentrations of 4,4'B. Cytotoxicity of 4,4'B on NHK was measured by neutral red release assay. Superoxide dismutase (SOD) activity and cell cycle were analyzed in exposed and nonexposed NHK cultures. Histological and immunohistological tests enabled evaluation of differentiation and proliferation effects in reconstructed-skin models. Results showed that significant cytotoxicity occurred after 5 to 11 days' exposure to 4,4'B concentrations of 10(-6)-10(-3) mol/L (IC50 between 10(-3) and 10(-4) mol/L 4,4'B after 11 days). Parallel modifications of SOD activity were recorded. Histological and immunohistological analysis revealed dose-related 4,4'B effects in reconstructed skin models. This involved abnormal terminal differentiation, connected with filaggrin expression, observed in skin models exposed to 10(-7) and 10(-6) mol/L 4,4'B. However, no modification of cell cycle or dysplasia was detected as a result of exposure to 4,4'B. Thus, 4,4'B appears to be cytotoxic for NHK, but as an isolated contaminant, and is unable to induce keratinocyte dysplasia in vitro. These preliminary results do not exclude a cocarcinogenic action of 4,4'B (with UVB for example).