A liposome assay reagent for determination of an analyte in a homogeneous immunoassay. The reagent includes a suspension of oligolamellar lipid vesicles containing encapsulated glucose-6-phosphate dehydrogenase (G6PD), at a specific activity of between about 1-10 units/.mu.mole vesicle lipid, and glucose-6-phosphate (G6P) at a concentration of at least about 5 mM. The encapsulated G6P protects the enzyme against inactivation on preparation, by reverse phase evaporation in the presence of organic solvent, and on storage as an aqueous suspension.