Exposure of humans to chemicals in beef or milk is part of almost all risk evaluation procedures carried out to reduce emissions or to remediate sites. Concentrations of substances in these livestock products are often estimated using log-log regressions that relate the biotransfer factor BTF to the octanol-water partition ratio K(ow). However, the correctness of these empirical correlations has been questioned. Here, we compare them to the mechanistic model OMEGA that describes the distribution of substances in organisms by integrating theory on chemical fugacity and biological allometry. OMEGA has been calibrated and validated on thousands of laboratory and field data, reflecting many chemical substances and biological species. Overall fluxes of water, food, tissue (growth), milk and stable substances calculated by OMEGA are within a factor of two from independent data obtained in experiments. Rate constants measured for elimination of individual compounds of a recalcitrant nature vary around the level expected from the model for output to faeces and milk. Both data and model suggest that biotransfer BTF of stable substances to beef and milk is independent of the octanol-water partition ratio K(ow) in the range of 10(3)-10(6). This contradicts empirical regressions including stable and labile compounds. As expected, levels of labile substances vary widely around a tentative indication derived from the model. Transformation and accumulation of labile substances remains highly specific for the chemical and organism concerned but depends weakly on the octanol-water partition ratio K(ow). Several possibilities for additional refinement are identified.