Rare-earth metal chelates used as fluorescent labels for immunoassays possess extremely long fluorescence lifetimes and permit the elective use of time-resolved detection. This is very important in fiber-optic fluorimetry, a technique that ordinarily exhibits large signal backgrounds from back-scattered radiation. With time-resolved detection to reject the back-scattered radiation, the limit of detection for Eu 2-naphthoyltrifluoroacetonate is 10(-12)M, nearly three orders of magnitude lower than for the fiber-optic measurement of the most common fluorescent label, fluorescein isothiocyanate. Commercially available reagents labeled with an europium chelate are used to demonstrate the potential utility of time-resolved fluorimetry in fiber-optic immunoassays. Rabbit immunoglobulin G (IgG) is covalently bonded to the distal end of quartz optical fibers prior to exposure to anti-rabbit IgG labelled with europium chelate. The limit of detection for the assay is approximately 0.1 mug/ml.