A simple single-particle model was developed for cross-flow microfiltration with microsieves. The model describes the cross-flow conditions required to release a trapped spherical particle from a circular pore. All equations are derived in a fully analytical way without any fitting parameters. For experimental verification of the model ultra-thin microsieves of uniform pore size and distribution were used. The release of trapped particles (polystyrene spheres and yeast cells) was determined by flux measurements as well as by in-line observation through a microscope. The results show that the model gives a fairly good indication of what cross-flow should be applied to keep the pores free for the conditions specified in this paper. In addition it provides us with a simple rule of thumb for the design of cross-flow modules for microsieves. It describes which geometrical demands have to be met to enable filtration without pore blocking, again for the conditions specified in this paper.