Temperature-sensitive drug release was examined using liposomes mixed with a copolymer of N-isopropylacrylamide (NIPAM) and acrylic acid [P(NIPAM-AA)] i.e., thermally responsive liposomes. P(NIPAM-AA) copolymers with transition temperatures of about 30, 33, 37, and 43 degrees C were synthesized by copolymerizing NIPAM and acrylic acid. Thermally responsive liposomes were prepared by mixing hydrophobically modified PNIPAM, or P(NIPAM-AA) with various liposomes, composed of egg phosphatidylcholine (PC), dimyristoylphosphatidylcholine (DMPC)/dipalmitoylphosphatidylcholine (DPPC) mixture (5: 5, w/w), DPPC, or distearoylphosphatidylcholine (DSPC). The release of a fluorescent marker, calcein, from liposomes was monitored by injecting the liposomal suspension at 17 degrees C into phosphate-buffered saline (PBS, pH 7.4) preadjusted to a temperature ranging from 20 to 46 degrees C. For liposomes of egg PC and DSPC, which do not undergo a phase transition during the temperature jump (17-->20-46 degrees C), the release temperature of the liposomes increased as the content of acrylic acid in the copolymers increased. The interaction between copolymer and lipid may induce the release of calcein at LCST of the copolymer. For DPPC liposomes, the release patterns were similar to those of egg PC and DSPC liposomes at 20-36 degrees C, where the phase transition of the liposomal membrane did not occur, while at 36-46 degrees C, where the phase transition of liposomal membrane occurred, the degree of release was almost the same. For DMPC/DPPC (5:5, w/w) liposomes, where the transition occurred below those of PNIPAMs, equally enhanced releases were observed as compared with PNIPAMs, even below the LCSTs of PNIPAMs. Thus, regardless of the occurrence of the transition of PNIPAMs, phase transition of DMPC/DPPC liposomes controlled the release of calcein.