Eight allergic patch test reactions, eight irritant skin reactions induced by 3% sodium lauryl sulphate and six normal controls were biopsied. Biopsies were immunohistochemically stained with a mouse monoclonal antibody to inducible nitric oxide synthase (iNOS), and staining was quantified by computerised image analysis. Human chondrocytes induced to express iNOS were used as a positive control. A significant increase in iNOS was found in both irritant and allergic contact dermatitis. There were no differences in the distribution of expression of iNOS. The antibody used was confirmed by Western blotting not to cross-react with the endothelial isoform of nitric oxide synthase (NOS) but did cross-react with a 150 kDa protein, which may be neuronal NOS or an isoform of neuronal NOS.