Abstract: A time-resolved fluoroimmunoassay （trFIA） for human HBsAg is described, in which antibodycoated microtiter strip-wells and Eu³⁺ chelate-labeled monoclonal antibody are used. These Eu³⁺ ions bound to the antibody molecules by means of the anhydrid of diethylenetriaminepentaacetic acid. The solidphase antibody is immobilized microtiter strip-wells in which serum samples were assayed in two-step procedure. The precision, sensitivty and clincal studies indicate the assay is accurate. The trFIA is technically simple, and takes a count time of only one second. In addition, the Eu³⁺ label which is nontoxic presents no wast-disposal problems and has a long shelf life.