We have examined the maximum sensitivity of a newly developed and optimized time-resolved fluorescence immunoassay system. The system, originally described elsewhere (Clin Biochem 1988:21:139-50), has undergone significant improvements in sensitivity through improvements of the labeled reagent used. We have chosen an alpha-fetoprotein (AFP) assay as a model and used monoclonal "capture" antibodies and monoclonal or polyclonal biotinylated antibodies in "sandwich-type" assay configurations. Streptavidin labeled with the europium chelator 4.7-bis(chlorosulfophenyl)-1,10-phenanthroline-2.9-dicarboxylic acid was used for detection. We can measure as few as 3 x 10(5) molecules of AFP with the optimized system. We have applied this assay to measure AFP in the serum of normal individuals after a 10-fold sample dilution. We conclude that this system is extremely sensitive and can be used in immunoassay or other applications where biotinylated reagents can be applied.