A time-resolved fluoro-immunoassay (TR-FIA) format is presented based on resonance energy transfer from visible emitting lanthanide complexes of europium and terbium, as energy donors, to semiconductor CdSe/ZnS core/shell nanocrystals (quantum dots, QD), as energy acceptors. The spatial proximity of the donor-acceptor pairs is obtained through the biological recognition process of biotin, coated at the surface of the dots (Biot-QD), and streptavidin labeled with the lanthanide markers (Ln-strep). The energy transfer phenomenon is evident from simultaneous lanthanide emission quenching and QD emission sensitization with a 1000-fold increase of the QD luminescence decay time reaching the hundred As regime. Delayed emission detection allows for quantification of the recognition process and demonstrated a nearly quantitative association of the biotins to streptavidin with sensitivity limits reaching 1.2 mu M of QD. Spectral characterization permits calculation of the energy transfer parameters. Extremely large Forster radii (R-0) values were obtained for Tb (104 angstrom) and Eu (96 angstrom) as a result of the relevant spectral overlap of donor emission and acceptor absorption. Special attention was paid to interactions with the varying constituents of the buffer for sensitivity and transfer efficiency optimization. The energy transfer phenomenon was also monitored by time-resolved luminescence microscopy experiments. At elevated concentration (> 10(-5) M), Tb-strep precipitated in the form of pellets with long-lived green luminescence, whereas addition of Biot-QD led to red emitting pellets, with long excited-state decay times. The Ln-QD donor-acceptor hybrids appear as highly sensitive analytical tools both for TR-FIA and time-resolved luminescence microscopy experiments.