Lab-on-a-chip microfluidic devices commonly manipulate biological fluids, which contain significant quantities of biomolecules, in particular proteins and DNA. This inevitably leads to the adsorption of these biomolecules to the surfaces of the devices through which the fluid flows. As the dimensions of these devices continue to decrease and approach the sub-micron range, this phenomenon will become more and more important. In this paper, we model the effect of protein adsorption on the flow of fluid through a microchannel, under the assumption that classical hydrodynamics applies. We predict that the adsorption of a thin layer of protein on the walls of a thin microchannel can significantly decrease the effective flow rate and velocity through the channel. Finally, we briefly mention the possibility that the increased wall roughness and modified wall chemistry, attributable to the deposition of the protein, could also lead to the modification of the flow, and in particular, may render the crucial “no-slip” wall condition invalid.