An object of the present invention is to provide a liposome and a liposome composition in which the release of a drug in blood is suppressed while accumulating at a target site such as a tumor, and the drug is quickly removed from the blood after sufficiently accumulating at the target site Is to provide things. According to the present invention, a liposome comprising at least a phospholipid to which a hydrophilic polymer is bound, a phospholipid having a negative charge in a pH range of 6 to 8, and a phospholipid that is electrically neutral in a pH range of 6 to 8. The liposome having an average carbon chain length of the phospholipid to which the hydrophilic polymer is bound is shorter than an average carbon chain length of a phospholipid that is electrically neutral in the pH range of 6 to 8. Provided.本発明の課題は、腫瘍などの標的部位に集積する間は血中での薬物の放出が抑制され、かつ標的部位に十分に集積した後に速やかに薬物が血中から除去されるリポソーム及びリポソーム組成物を提供することである。本発明によれば、親水性高分子が結合したリン脂質、ｐＨ６～８の領域で負電荷を持つリン脂質、及びｐＨ６～８の領域で電気的に中性であるリン脂質を少なくとも含むリポソームであって、上記の親水性高分子が結合したリン脂質の平均炭素鎖長が、上記のｐＨ６～８の領域で電気的に中性であるリン脂質の平均炭素鎖長よりも短い、上記リポソームが提供される。
