Similar to chaperonins from other sources, intact chaperonin from Escherichia coli (GroEL) exists as a tetradecamer, and the ability to promote folding of other proteins has been considered to be dependent on this oligomeric structure. However, the peptide fragments of GroEL of molecular size 34-50 kDa, which are produced by limited proteolysis of monomeric GroEL and are unable to assemble into an oligomer, retain the ability to promote folding of rhodanese even though the yield of productive folding is lower than the intact GroEL/GroES/ATP system. This promotion by truncated GroEL obeys rapid kinetics and does not require GroES and ATP.