Multixenobiotic resistance (MXR) is a mechanism analogous to the mammalian multidrug resistance (MDR) phenotype, whereby, simultaneous resistance is conferred against the intracellular accumulation of structurally and functionally diverse, natural, endogenous and environmental toxicants. Expression of P-glycoproteins (P-gp), ATP-dependent transporters encoded for by the mdr1 gene that have been implicated in this xenobiotic efflux mechanism, have previously been detected in normal teleost tissues involved in a secretory, absorption or a barrier function. The presence of these proteins in the epidermis of fish species has not to our knowledge previously been investigated. In the present study, primary cultures of epidermis from the rainbow trout Oncorhynchus mykiss were employed to investigate whether an MXR mechanism is functional in the epidermis of fish. The efflux of the fluorescent mdr1 substrate rhodamine 123 from the cells was significantly inhibited by verapamil, a compound known to interfere with P-gp mediated transport. The cultured epidermal cells were also observed to accumulate this fluorescent dye in a verapamil sensitive manner, thus indicating the presence of an mdr1-like mechanism. Immunocytochemical analysis, using a monoclonal antibody (JSB 1) directed against a conserved cytoplasmic P-gp epitope, also demonstrated the presence of P-gp-like proteins. Sediment elutriate extracts were employed as models of environmental complex mixtures to evaluate the potential of the epidermal cultures to discriminate between samples of varying contaminant burden using MXR activity as an endpoint. The induction of P-gp expression was found to be in accordance with the level of contamination detected in the sediments from which the elutriates were extracted. The findings of the functional study also demonstrated that environmental pollutants, which interfere with P-gp function, could be identified using this model. (c) 2005 Elsevier B.V. All rights reserved.