Microorganisms are a source of protein with catalytic and/or biological activity, which are of increasing commercial interest for applications in industry or therapy. for the isolation of intracellular products cell disruption is necessary. In principle, chemical, biological, or physical means may be employed to release proteins from cells. These different approaches are reviewed with special emphasis on scale-up and possible industrial operation. Mechanical devices have been improved considerably in recent years and appear most universally suited to cell disintegration. Chemical extraction or enzymatic lysis offers improved selectivity but requires individual procedures for each product. For a final process design, product yield and cost of the unit operation must be considered as well as the possible implications for the subsequent steps in product recovery, especially on solid/liquid separation