Polyclonal antibodies raised against partly purified aminopeptidase were specific to cell-free extracts of Lactobacillus casei subsp. pseudoplantarum UL 137, and did not cross react with any other proteins in Cheddar cheese, at least during the first week of maturation. A sandwich enzyme-linked immunosorbent assay (ELISA) was developed in order to quantify cell-free extracts in the cheese curd, and was also used to determine the efficiency of encapsulation of cell-free extracts in liposomes. This method was very sensitive and exhibited a detection limit of ∼10 μg total protein/g cheese and ∼1 μg total protein/ml liposome suspension. The efficiency of encapsulation of cell-free extracts into liposomes was ∼55–60%. The retention of liposome-encapsulated cell-free extracts was ∼14 times that of non-encapsulated extracts.