The process of fusion of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes in the presence of stimuli responsive copolymers (poly(N-isopropylacrylaniide-co-methacrylic acid) and poly(N-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylamide)) or proteins ((x-chymotrypsin (alpha-CT), bovine carbonic anhydrase (CAB), and p-galactosidase (p-gal)) was quantitatively characterized under the stimuli by varying pH and temperature. After the liposomes were exposed to the specific pH and heat stimuli in the presence of the stimuli responsive polymers or proteins, the percentage of the fusion was determined by using two kinds of liposomes entrapping cobalt-calcein and EDTA (cobalt-calcein method). In the presence of stimuli responsive copolymers, the percentage of fusion was increased to 20% above the phase transition condition of the copolymer (above 37 degreesC and below pH 5.7). The percentage of fusion in the presence of proteins was varied under the stimuli, depending on the type of proteins. In the case of the alpha-CT, the maximal percentage of fusion was only 8%. The addition of the CAB and p-gal improved the percentage of fusion to 15 and 20%, respectively, by selecting the optimal stimuli conditions. Although some disagreements were observed in the region of strong acidic conditions, it was found that the increase of the percentage of fusion was well corresponding with the increase of the membrane fluidity of the POPC liposome, followed by the increase of the local hydrophobicity of proteins or copolymers under the stimuli. Based on the above results, a possible model for the stimuli-induced fusion of POPC liposome membranes was finally presented. (C) 2002 Elsevier Science B.V. All rights reserved.