The GroEL molecular chaperone of Escherichia coli and its cofactor GroES are highly conserved, and are required for the folding of many proteins. Most but not all bacteria express single GroEL and GroES proteins. Rhizobium leguminosarum strain A34 encodes three complete operons encoding homologues to GroEL and GroES. We have used circular dichroism and measurement of ATPase activity to compare the stabilities of these chaperonins after expression in and purification from E coli. Significant differences in the stabilities of the proteins with respect to denaturant and temperature were found. The proteins also differed in their ability to refold denatured lactate dehydrogenase. This study, the first to compare the properties of three different GroEL homologues from the same organism, shows that despite the high degree of similarity between different homologues, they can display distinct properties in vitro. (C) 2004 Elsevier Inc. All rights reserved.