AbstractA novel protein purification process using affinity‐ligand‐modified liposomes and membrane ultrafiltration is described. The feasibility of the process was tested using trypsin as the model protein and p‐aminobenzamidine (PAB) as the affinity ligand for trypsin. The affinity liposomes were prepared by covalently attaching PAB to the surface of small unilamellar liposomes via the hydrophilic spacer arm diglycolic acid. The liposomes were comprised of dimyristoyl phosphatidyl choline, cholesterol, and dimyristoyl phosphatidyl ethanolamine to which the diglycolic acid was attached. The equilibrium binding constant between trypsin and immobilized PAB was shown to be dependent on the PAB density of the liposome surface. Bound trypsin was eluted from the liposomes by the trypsin inhibitor benzamidine. Trypsin was purified from a trypsin/chymotrypsin mixture and from one of its naturally occurring sources, porcine pancreatic extract. A recovery yield from the crude mixture of 68% was obtained with a trypsin purity of 98%. The affinity‐modified liposomes were stable in the complex mixture and retained their trypsin binding capacity after multiple adsorption/elution cycles over a 30‐day period.