The stability of an SR398/GroES chaperonin complex was examined. As was expected, based on the finding of previous studies, the SR398/GroES complex was extremely stable in the presence of an excess amount of free adenosine 5'-[γ-thio]triphosphate (ATPγS) or adenosine 5'-(β,γ-imido)triphosphate (AMPPNP). However, the complex was not stable in the absence of nucleotides. These results indicate that ATPγS and AMPPNP repeatedly associated to and dissociated from the complex in a non-cooperative manner. This nucleotide exchange did not induce the dissociation of GroES and substrate from SR398, suggesting the importance of the cooperative dissociation of nucleotides from the cis-ring to release GroES and substrate proteins in the GroEL/GroES reaction cycle.