CHAPERONINS are ubiquitous multisubunit toroidal complexes that aid protein folding in an ATP-dependent manner(1-6). Current models of folding by the bacterial chaperonin GroEL depict its role as unfolding and releasing molecules that have misfolded, so that they can return to a potentially productive folding pathway in solution(7,8). Accordingly, a given target polypeptide might require several cycles of binding and ATP-driven release from different chaperonin complexes before reaching the native state. Surprisingly, cycling of a target protein does not guarantee its folding, and we report here that unfolded beta-actin or alpha-tubulin both form tight complexes when presented to either GroEL or its mitochondrial homologue, and both undergo cycles of release and rebinding upon incubation with ATP, but no native protein is produced. We conclude that different chaperonins produce distinctive spectra of folding intermediates.