The conceptual model on which chemical assessment of pollutants is based is flawed. The assumption in ecotoxicology -that pollutants cross the biological membrane only by passive diffusion of their solute phase (diffusion model) and thus, that only water-soluble pollutants are biologically available -is inconsistent with the biologists' understanding of the role and functioning of the biological membrane. The biological membrane both isolates organisms, cells and organelles from their external environments and regulates cross-membrane trafficking of polar and non-polar substances. Trafficking regulation is a function of proteins dissolved within the membrane. An alternative protein model for pollutant uptake and elimination is proposed that provides a credible explanation of how solid-phase pollutants, such as those bound to aquatic sediments and soils, may be readily incorporated by living organisms. Current chemical testing is likely to underestimate the risk posed to organisms by sediment and soil-bound pollutants. New techniques for assessing the bioavailability and impacts of pollutants, based on the protein model, are urgently needed.