The chaperone-like activity of a nanogel of cholesteryl group-bearing pullulan (CHP) was compared with that of GroEL for refolding acid-denatured green fluorescent protein (GFP). The refolding of denatured GFP was carried out by dilution of acid-denatured GFP in the presence of the nanogel or GroEL. GFP fluorescence was increasingly repressed with increases in the concentration of the CHP nanogel or GroEL added to the dilution buffer. The concentrations of 50% inhibition of recovery of GFP fluorescence were 0.03 microM (GroEL) and 0.08 microM (CHP nanogel), respectively. The refolding was resumed by the addition of ATP into the GroEL (0.20 microM) system or by the addition of methyl-beta-cyclodextrin into the nanogel (0.20 microM) system. In the nanogel-GFP system, about 90% of the intensity was recovered within 10 min. The half time (t(1/2)) for refolding in the CHP nanogel system (36 s) is almost equal to that of the natural chaperone GroEL-GroES system.