The preparation and immunoassay performance of europium-labeled streptavidin is described. The Eu-streptavidin conjugate can serve as a general detection reagent in time-resolved fluoroimmunoassays (TRFIA). The usefulness of such a strategy has previously been demonstrated with the Eu3+ chelate 4,7-bis(chlorosulfophenyl)-1,10-phenanthroline-2,9-dicarboxylic acid (Diamandis et al. 1988). In this report the conjugation of streptavidin was accomplished with the Eu3+ chelate of N1-(p-isothiocyanatobenzyl)diethylenetriamine-N1,N3,N3(+)-tetraace tic acid, using a 50 M excess of the label. The conjugation ratio of Eu3+/streptavidin was 16. The use of the Eu-streptavidin reagent in a two-site immunometric assay to measure human recombinant interleukin-3 in human plasma, showed that the useful range of the assay was 20-25,000 pg/ml.