On the basis of a previous report on the occurrence of water-soluble cyanophycin (CGP. cyanophycin granule polypeptide) in a recombinant strain of Escherichia coli expressing the cyanophycin synthetase (CphA) of Desulfitobacterium hafniense published by others, the conditions of its production were investigated in this study. Although the incubation temperature, aeration level, and NaCl concentration during cultivation had effects on the in vivo production of water-soluble CGP, it could be isolated as a major variant irrespective of the cultivation conditions. The occurrence of the soluble variant was also not dependent on the E. coli host or on the origin of cphA. Furthermore, it was shown that water-insoluble CGP can be in vitro solubilized to extents of up to about 80% (w/w) in solutions of different inorganic salts such as LiCl, NaCl, KCl, RbCl, KBr, MgCl2, or CaCl2. Evidence was obtained that the salt ions bind tightly to CGP. If the ions were not removed from the salt solution by dialysis or dilution, the CGP remained stable in solution. This method to solubilize water-insoluble CGP could also be applied to high concentrations of the polymer. CGP that remained insoluble after the first treatment could only marginally be solubilized in following treatments. The polydisperse CGP molecules were solubilized to the same extent over the whole molecular weight range with no preference of a particular molecular weight.