Hsp104 is a stress tolerance factor that promotes the reactivation of heat-damaged proteins in yeast by an unknown mechanism. Herein, we demonstrate that Hsp104 functions in this process directly. Unlike other chaperones, Hsp104 does not prevent the aggregation of denatured proteins. However, in concert with Hsp40 and Hsp70, Hsp104 can reactivate proteins that have been denatured and allowed to aggregate, substrates refractory to the action of other chaperones. Hsp104 cooperates with the chaperones present in reticulocyte lysates but not with DnaK of E. coli. We conclude that Hsp104 has a protein remodeling activity that acts on trapped, aggregated proteins and requires specific interactions with conventional chaperones to promote refolding of the intermediates it produces.