Egg yolk phosphatidylcholine liposomes modified with a copolymer of N-acryloylpyrrolidine and N-isopropylacrylamide having a lower critical solution temperature at ca. 40 degrees C were prepared and an effect of temperature on their interaction with CV1 cells was investigated. The unmodified liposomes were taken up by the cells approximately to the same extent after 3 h incubation at 37 and 42 degrees C. In contrast, uptake of the polymer-modified liposomes by CV1 cells decreased slightly at 37 degrees C but increased greatly at 42 degrees C, compared to the unmodified liposomes. Proliferation of the cells was partly prohibited by the incubation with the unmodified liposomes encapsulating methotrexate at 37 and 42 degrees C. The treatment with the polymer-modified liposomes containing methotrexate at 37 degrees C hardly effected the cell growth. However, the treatment at 42 degrees C inhibited the cell growth completely. It is considered that the highly hydrated polymer chains attached to the liposome surface suppressed the liposome-cell interaction below the lower critical solution temperature of the polymer but the dehydrated polymer chains enhanced the interaction above this temperature. Because interaction of the polymer-modified liposomes with cells can be controlled by the ambient temperature, these liposomes may have potential usefulness as efficient site-specific drug delivery systems.