Inclusion bodies (IBs) are micron-sized solid protein particles that form within the cytoplasm of certain host cells such as Escherichia coli following overexpression of a protein. IBs are comprised primarily of the recombinant protein of interest. Some contaminants including nonproduct protein, nucleic acids, and cell-envelope contaminants can also be incorporated into the granules. However, it is believed that the majority of contaminants actually adhere to the IB surface following release from the cytoplasm during processing (1). This indicates that IB formation in vivo is a rather specific process that offers certain advantages for downstream processing. Specifically, the protein of interest already exists in a relatively pure state as a small granule that can be recovered by physical separation from nonassociated contaminants. Of course, if a suitably efficient protein refolding strategy is not available (2), then any gains achieved through inclusion body formation may be easily lost.