Labelling of streptavidin with a fluorogenic europium ion was optimized with the aim of obtaining a universal, stable and highly sensitive non-isotopic label for time-resolved fluorometric immunoassays (TR-FIA) based on dissociative fluorescence enhancement (DELFIA). Even the conjugation of all the free amino groups of streptavidin with Eu chelates had only a minor effect on the binding capacity of the protein or its affinity. The labelled streptavidin was evaluated in indirect time-resolved immunofluorometric assays of human follicle and thyroid stimulating hormones (FSH and TSH). The interassay imprecision was below 3% within the concentration range from 2.5 to 94 U/l for the FSH samples and below 5% in the range from 2.4 to 35 mIU/l for the TSH samples. The detection limits of the assays for FSH and TSH were 0.05-0.10 U/l and 0.01-0.025 mIU/l, respectively, when a CV of 15% was regarded as the acceptable upper limit of imprecision. The results obtained by the indirect assays correlated closely with those obtained by corresponding direct sandwich assays. The model assays demonstrated the utility of Eu-labelled streptavidin as a universal reagent for immunoassays requiring a wide dynamic range and high sensitivity.