In order to understand the role of ATP hydrolysis of the chaperonin GroEL during protein folding, we have studied GroEL-GroES complex formation in the presence of ATP or ADP by using capillary electrophoresis and surface plasmon resonance. Capillary electrophoresis analysis showed that the GroEL 14-mer and GroES 7-mer formed a 1:1 complex in the presence of ATP. In the pressence of ADP, both the association and dissociation rates of the complex were slower by about one order of magnitude than the rates in the preence of ATP at 25°C. The implications of such a stable complex on the overall mechanism of chaperonin function are discussed.