The importance of chaperonin-protein interactions has been investigated by analyzing the refolding of the barley chymotrypsin inhibitor 2 in the presence of GroEL. The chaperonin retards the rate of refolding of wild type and 32 representative point mutants. The retardation of the rate drops to a finite level at saturating concentrations of GroEL, being lowered by a factor of 3-100, depending on the mutation. It is seen qualitatively that truncation of large hydrophobic side chains to smaller side chains weakens binding. Analysis of the magnitude of the rates of retardation shows further that hydrophobic and positively charged side chains tend to interact favorably with GroEL whereas negatively charged side chains tend to repel. There is an inverse correlation between the strength of hydrophobic interactions and the rate constant for refolding of the GroEL-complexed protein: the better the binding, the slower the folding. This shows directly that hydrophobic (and other favorable) interactions between the chaperonin and substrate are weakened during the refolding process and implies that unfolding can be catalyzed by the gain of such interactions.