A polymerized liposome (PLS) was prepared using a synthesized phospholipid with a diacetylene moiety in the hydrophobic chain and an amino group in the hydrophilic head. The PLS was used as a novel ligand carrier for affinity precipitation of proteins because it showed a reversibly precipitable property on salt addition and removal. Soybean trypsin inhibitor (STI) was easily immobilized on the PLS by a one-step carbodiimide reaction. The PLS showed no nonspecific adsorption of proteins. It had a large ligand coupling capacity, and then a large adsorption capacity for trypsin after STI immobilization. The PLS with immobilized STI was recycled three times for the purification of trypsin from a crude pancreatic extract. Although the degree of purification was com promised by the impurity of the STI employed, in each run the purification factor reached about 6 and more than 80% of trypsin activity was recovered. The results indicated that the PLS was a potential ligand carrier for affinity precipitation of proteins.