Recent reports have shown that synthesis of certain recombinant proteins in Escherichia coli results in the production of intracellular inclusion bodies. These studies have not analyzed the structure of the inclusion body especially regarding the intermolecular forces holding it together. We have examined structural aspects of inclusion bodies made in E. coli as a result of high level expression of the eukaryotic protein, calf prochymosin. Prochymosin is a monomeric protein containing three disulfide bridges. It was expressed at up to 20% of cell protein from a plasmid containing the E. coli tryptophan promoter, operator and ribosome binding site. Proteins in the inclusion bodies were analysed by Western blotting of SDS-polyacrylamide gels. When experiments were done using conditions which preserved the in vitro state of thiol groups, inclusions were shown to be composed of multimers of prochymosin molecules which were interlinked partly by disulfide bonds. The inclusion bodies also contained a high concentration of reduced prochymosin. The presence of intermolecular disulfides probably contributes to the difficulty of solubilizing recombinant prochymosin during its purification from E. coli.