Unlike Escherichia coli chaperonins, a chaperonin (cpn) from a thermophilic bacterium, Thermus thermophilus, consisting of homologues to GroEL (cpn 60) and GroES (cpn 10) is co-purified as a large complex. Thermus chaperonin shows a bullet-like shape in the side view seen by electron microscopy, and antibody against cpn 10 binds only to the round side of the bullet. We conclude that a single cpn 60-heptamer ring with two stripes stacks into two layers and a cpn 10 oligomer binds to one side of the layers. The purified Thermus chaperonin contains endogenously bound ADP, and incubation with ATP causes a partial dissociation of chaperonin into cpn 60 monomers and a cpn 10 heptamer. The effect of Thermus chaperonin on protein refolding upon dilution from guanidine HCl is different at three temperature ranges. At high temperatures above 55 degrees C, where the native proteins are stable but their spontaneous foldings fail, the chaperonin induces productive folding in an ATP-dependent manner. At middle temperatures (25-55 degrees C) where spontaneous foldings of the enzymes occur, the chaperonin slows down the rate of folding without changing the final yield of productive folding. At lower temperatures below 25 degrees C where spontaneous foldings also occur, the chaperonin arrests the folding even in the presence of ATP. When a solution of relatively heat labile protein is incubated at high temperatures, and then residual activity of the protein is measured at its optimal temperature after incubation with ATP, the temperature that causes irreversible heat denaturation of the protein is elevated about 10 degrees C by inclusion of Thermus chaperonin in the solution. Furthermore, once the folding intermediate of a protein is captured by Thermus chaperonin, it retains the ability to resume productive folding even after exposure to the otherwise denaturing high temperature. These results indicate that during heat denaturation proteins assume the common structure which is recognizable by the chaperonin. Finally, a `folding intermediate reservoir' model to explain the effect of chaperonin is proposed, and is compared with a `marsupium' model.