The ultrafiltration of a single protein (beta-lactoglobulin) solution was compared with a mixture of yeast extract and beta-lactoglobulin to show that, even when all the compounds of a complex biological solution interfere with the results of filtration, it can estimate its performance just using some major assumptions connected to a mass-transfer model. Preliminary experiments based on these assumptions showed that the filtration of a single rejected protein like beta-lactoglobulin builds a dynamic layer at the wall, the selectivity and resistance of control by the protein concentration and formation of this dynamic membrane. Adding yeast extract, a complex biological medium, can alter this layer. But at neutral pH, away from the protein isoelectric point, these modifications are weak and their effects more negligible than those arising from a modified concentration. Using these assumptions, a model based on the theory of thermodynamics of irreversible processes is proposed that provides the correct description of selectivity and flux during the filtration of such complex and badly known mixtures.