Purified preparations of phosphatidylcholine (lecithin): cholesterol acyltransferase (EC 2.3.1.43), were injected into goats to produce antisera reacting with this enzyme. The antisera and the gamma-globulin derived thereform were examined by the technics of immunodiffusion, immunoelectrophoresis and immunoinhibition of the enzyme. The antisera gave no precipitation lines with human high density lipoproteins (HDL) and human low density lipoproteins (LDL). A weak antibody titer towards human serum albumin was noted only after prolonged immunization. The enzymatically active band isolated from acrylamide gels gave a single arc in immunodiffusion and immunoelectrophoresis. The gamma-globulin derived from the antisera inhibited human phosphatidylcholine:cholesterol acyltransferase activity.