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Europium has been used as a non-radioactive marker in immunoassays as this metal can be detected with high sensitivity by time-resolved fluorometry. In this work streptavidin labeled with europium was used to detect biotinylated probes in a sandwich nucleic-acid hybridization assay with microtitration strips as the solid phase. pBR 322 plasmids were detected with a sensitivity of 4 x 10(5) molecules. As the sample is added in solution in sandwich hybridization, fast and simple sample pre-treatment can be used without encountering background problems. The method was applied to test bacterial samples of uropathogenic Escherichia coli strains for the presence of the beta-lactamase gene.