A new solid-phase immunoassay, time-resolved fluoroimmunoassay (TR-FIA), for rubella antibody was developed. The test used polystyrene beads coated with rubella antigen as the solid phase and a chelate of the rare earth metal europium as fluorescent label. A fast light pulse from a xenon flash lamp was used to excite the label, and after a 400-mus delay time the emission fluorescence was measured for 500-mus at 1-ms intervals during a total counting time of 1 s. Background fluorescence of short duration caused by fluorescent serum components and scattering could be eliminated by including the delay time. The TR-FIA was compared with hemagglutination inhibition, single radial hemolysis, and two types of radioimmunoassay (RIA) (a commercial RIA [GammaCoat] and a noncommercial RIA [T-RIA]) by using 60 serum specimens from patients with remote rubella infection. Overall agreement of TR-FIA with hemagglutination inhibition and GammaCoat was 96.7%, with single radial hemolysis 98.3%, and with T-RIA 100%. Linear regression coefficients varied from 0.83 to 0.94, the best being obtained with single radial hemolysis and T-RIA. TR-FIA was also found to be suitable for the diagnosis of acute infections, as significant increases of antibody level were detected in all 30 paired serum specimens tested from patients with an acute rubella infection. Sensitivity and specificity comparable to those of RIA and enzyme immunoassay were obtained with TR-FIA. Furthermore, the advantage that TR-FIA has over RIA is that it incorporates a nonisotopic and stable label; its advantage over EIA is that it is easier to standardize because no additional reaction with substrate is required.