The invention relates to a process for purifying and/or concentrating inclusion bodies (refractile bodies) in a highly concentrated particle-containing solution, in which process the inclusion body-containing suspension is directed tangentially past one or more semipermeable membranes, so that the inclusion bodies are retained behind the membranes and that substances having a relatively small molecular weight may pass through the membrane and/or be adsorbed to the membrane, and a purified and/or concentrated inclusion body suspension is obtained. In the same manner, cell wall particles are removed from a inclusion body suspension. The invention further relates to the use of a crossflow microfiltration unit for purifying and/or concentrating inclusion bodies in a suspension and for removing cell wall particles from a inclusion body preparation.