Marine mammals can accumulate environmental contaminants in their blubber at concentrations harmful to laboratory animals. Induction of the cytochrome P450 1A1 (CYP1A1) enzyme is widely used as a biomarker of exposure and molecular effects in animal species, yet the validity of this biomarker has not been established in marine mammals. In vivo studies are generally precluded in these protected species, but skin biopsies (epidermis and dermis) can be collected in a minimally invasive way. We developed an in vitro assay using skin biopsy slices to examine CYP1A1 protein induction in marine mammals in response to chemical exposure. Skin biopsies from sperm whale (Physeter macrocephalus) were exposed for 24 h to beta-naphthoflavone (BNF), a prototypical CYP1A1 inducer, and CYP1A1 induction was detected by immunohistochemical staining in endothelial cells, smooth muscle cells, and fibroblasts. Biopsy slices were exposed to a range of BNF concentrations (0.6-600 microM), and a significant concentration-effect relationship was observed in both endothelial and smooth muscle cells (p = 0.05). This is the first study using skin biopsy slices to examine exposure of cetacean tissue to a CYP1A1 inducer. It demonstrates a causal relationship between chemical exposure and CYP1A1 induction and therefore validates the use of CYP1A1 expression in skin biopsies as a biomarker in cetaceans. Our protocol can be adapted to the investigation of chemicals, mixtures, concentrations, incubation times, or biological endpoints of choice. This should prove particularly relevant for these and other protected species that cannot be studied in the laboratory.