Immunoassay by fluorescence energy transfer from a europium chelate in liposome to allophycocyanin (APC) was demonstrated. Streptavidin or antibody to biotin was bonded to the liposome containing the europium chelate of 2-naphthoyltrifluoroacetone in the bilayer. When the biotin bonded to APC (APC–BT) was added to the prepared liposomes and the long-lived fluorescence (λ_ex 336 nm, λ_em 665 nm, delay time 0.05 ms, gate time 3.9 ms) was measured by a flow system, the fluorescence energy of the europium chelate was found to be transferred to APC–BT and the long-lived fluorescence intensity to increase linearly as the concentration of APC–BT (1–10 µg ml^–1) increased. Further, the intensity decreased competitively with the concentration of biotin (0.1–100 µM) when biotin was added to the liposome solution containing a constant concentration of APC–BT.