Chaperonins participate in the facilitated folding of a variety of proteins in vivo. To see whether the same spectrum of target proteins can be productively folded by the double-ring prokaryotic chaperonin GroEL-GroES and its single-ring human mitochondrial homolog, Hsp60-Hsp10, we expressed the latter in an Escherichia coli strain engineered so that the groE operon is under strict regulatory control. We found that expression of Hsp60-Hsp10 restores viability to cells that no longer express GroEL-GroES, formally demonstrating that Hsp60-Hsp10 can carry out all essential in vivo functions of GroEL-GroES.