The discovery and properties of the plastid chaperonin are described. This chaperonin is implicated in the folding and assembly of the enzyme ribulose bisphosphate carboxylase-oxygenase and in the folding of proteins imported into plastids from the cytosol. The plastid chaperonin appears to be unique in that it contains two distinct types of 60 kDa polypeptide, whereas only a single subunit type has been reported for the bacterial and mitochondrial chaperonins. The plastid chaperonin polypeptides are encoded by nuclear genes which are subject to complex regulation.