We report a novel homogeneous immunoassay system involving protein-bearing liposome-encapsulated carboxyfluorescein as a release marker. We applied this system to determine protein antigens, e.g., ferritin, in human serum samples by a sandwich-type assay. Liposomal lysis was observed in many samples, even though no second antibody was added to the reaction mixture. We demonstrated that the functional groups used to immobilize an antibody on liposomes are related to this phenomenon. Stable liposomes in human sera were prepared by incorporating bromoacetyl groups instead of the dithiopyridyl groups used previously. A good correlation (y = 0.98x - 8.81, r = 0.98, Sx/y = 66.9, range approximately 10-2000 micrograms/L) with results by RIA was obtained in the ferritin measurement of 53 patients' sera by using these liposomes.