Cysteine 518 of the molecular chaperonin cpn60 (groEL) has been replaced with serine (C518S) by site-directed mutagenesis. The resulting mutant chaperonin protein is still functional and it can: (a) arrest the spontaneous folding of rhodanese in the absence of GroES and ATP, (b) assist refolding of the enzyme rhodanese in the presence of GroES and ATP/Mg2+, and (c) permit the urea-induced release and refolding of rhodanese from its complex with cpn60. ATP/Mg2+, alone, could discharge active rhodanese from cpn60 complexes formed with either wild type or C518S. In contrast with wild type cpn60, C518S has: (a) reduced stability of its quaternary structure, (b) reduced ability to reassemble tetradecamers after dissociation by urea; (c) reduced ATPase activity; and (d) more easily exposed hydrophobic surfaces. The data suggest that replacement of Cys-518 with Ser in cpn60 destabilizes its oligomeric structure, but there is no significant effect on cpn60 function or the stability of the monomers formed in urea.